A small cuckoo nest, son agrees (7)
I believe the answer is:
assents
'agrees' is the definition.
(assenting is a kind of agreeing)
'a small cuckoo nest son' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'cuckoo' indicates an anagram (I've seen this in other clues).
'son' becomes 's' (genealogical abbreviation for son).
'nest' is an anagram of 'sent'.
'a'+'s'+'sent'+'s'='ASSENTS'
(Other definitions for assents that I've seen before include "Expresses agreement" , "Accedes, acquiesces" , "Says yes" , "Gives nod" , "Agrees, complies" .)