Amphibian beginning to wriggle in trap (4)
I believe the answer is:
newt
'amphibian' is the definition.
(newt is a kind of amphibian)
'beginning to wriggle in trap' is the wordplay.
'beginning to' suggests taking the first letters.
'in' is an insertion indicator.
'trap' becomes 'net' (net is a kind of trap**).
The first letter of 'wriggle' is 'w'.
'w' placed within 'net' is 'NEWT'.
(Other definitions for newt that I've seen before include "found in pond!" , "Pond creature" , "Semiaquatic amphibian" , "Medical worker" , "Went (anag.)" .)