Watch cuckoo line nest (8)
I believe the answer is:
sentinel
'watch' is the definition.
('sentinel' can be a synonym of 'watch')
'cuckoo line nest' is the wordplay.
'cuckoo' is an anagram indicator (I've seen this in other clues).
'line'+'nest'='linenest'
'linenest' with letters rearranged gives 'SENTINEL'.
(Other definitions for sentinel that I've seen before include "Guard who stands and keeps watch" , "Look out" , "Watcher on guard" , "Guard - ten lines (anag)" , "One watches" .)