The other unclued entries, in pairs around 3D, provide the key words in a definition of 11D.
 

Across

 
1    Nouns hidden in secret and manuscript sentences (8)
10    Sea beast who I’ll tape somehow (10, two words)
13    Goat starts to gnaw at one fish (8)
16    ‘Nameless’ ‘noodle’ could be ‘oodle’? (5)
17    Put into Debrett’s, having been no Lord to begin with sadly (7)
18    Where nun is to occupy (7)
20    Neat cube, square down under (8, two words)
25    Brazil won regularly here? (3)
26    I propose nothing for holding referee back (7)
28    Upset how big-headed someone is? (7)
29    Head teachers (3)
31    Donkey began to drop stone reclaimed for the farm (8)
34    Variant of ‘us’ (refl.) possibly (7)
36    Endless record about piano shops (7)
40    High wind requiring high gear in changing lane (8, two words)
42    Starter for meal from menu’s chosen badly – amazing old things (10)
43    Like poetic feet, ironically (6)
44    Curse river vehicle that’s broken down (8)
 

Down

 
1    Able to gain purchase with cents? (6)
2    Awful noise grows, not having united members of flock (8)
4    Guru was converting me (5)
6    Relative simpleton (4)
7    Turns round food perhaps flipped – like these? (11)
8    Extra answer marks here? (4)
9    Drive tube about with compounds (8)
14    I must become between 7 and 10, elementally (4)
15    What’s indeed ending in December? (4)
19    Handle spades totally lacking in handles (4)
21    Cooks deer (4)
22    Bill makes a fortune with these charged items (4)
25    Groom’s mate wanting leader to take horse (4)
27    Eulogist who rang the changes? (8)
30    Emperor on vacation teases a king (4)
31    A bit of greenery one’s found around lake (4)
32    One making a report sounded out a detective (6)
33    Seal is terribly famous pet (6)
37    Keep barking? I may (4)
38    Hindu god admitted to winding atheists up (4)
 
A first prize of £30 for the first correct solution opened on 5 January. There are two runners-up prizes of £20. (UK solvers can choose to receive the latest edition of the Chambers Dictionary instead of cash— ring the word ‘Dictionary’.) Entries to: Crossword 2191, The Spectator, 22 Old Queen Street, London SW1H 9HP. Please allow six weeks for prize delivery.