The unclued four-letter words can be paired in a particular way to form the remaining unclued lights (one of two words).
 

Across

 
5    Be in contact with a jock’s love-in (6)
10    Isolation of actors Chaney and Wallach by headland (10)
12    Scar on saint is something of a problem (6)
13    Still the champion, having downed the sticky bun first (8)
16    Little US heads (5)
17    Hermes’ footwear for 100 sene at ‘The Valley’ (7)
18    Does the gallery own something like Trident? (7)
20    European calls young lad back (8)
25    German listener returned letter (3)
26    Vessel...