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...