Wave beginning to enter wife’s wader (6)
I believe the answer is:
curlew
'wader' is the definition.
(curlew is a kind of wader)
'wave beginning to enter wife's' is the wordplay.
'wave' becomes 'curl' (synonyms).
'beginning to' suggests taking the first letters.
The first letters of 'enter wifes' is 'ew'.
'curl'+'ew'='CURLEW'
(Other definitions for curlew that I've seen before include "Wading bird with long down-curved beak" , "Wader with curved bill" , "Bird with long down-curved bill" , "Curve-beaked bird" , "Wader with downcurved bill" .)