Bird’s cry cheers a tot at sea (13)
I believe the answer is:
'bird's' is the definition.
(oystercatcher is a kind of bird)
'cry cheers a tot at sea' is the wordplay.
'at sea' indicates an anagram.
'crycheersatot' anagrammed gives 'OYSTERCATCHER'.
(Other definitions for oystercatcher that I've seen before include "Carpenter, for one" , "Pied coastal wading bird" , "Bird of the seashore" , "Black and white wader" , "flier with large bill" .)