Unclued lights (two hyphened) are words (all but one of which are in ODQ) from a poem whose title appears in the completed grid. Two clued lights combine to give the poet’s surname and must be shaded.
 

Across

 
1    Brass hat beginning to eat brassica (4)
9    Gem from ring put on partner (4)
11    Break commandment, shattering true ideals (10)
13    Mademoiselle’s protest (4)
15    Clouds covering Dutch centres (6)
17    City guides in conversation (5)
18    Frost’s poems? (5)
20    Mantle’s everything one crony cherishes (7)
21    Killing time, polecats mauled mollusc (7)
24    Crest of bird part adrift (5)
25    Tower bedroom of good height (5)
27    Greatest possible bird...