Bird — and cat beginning to track it (6)
I believe the answer is:
tomtit
'bird' is the definition.
(tomtit is a kind of bird)
'cat beginning to track it' is the wordplay.
'cat' becomes 'tom' (a tom is a male cat).
'beginning to' suggests taking the first letters.
The first letter of 'track' is 't'.
'tom'+'t'+'it'='TOMTIT'
'and' acts as a link.
(Other definitions for tomtit that I've seen before include "Popular name for a songbird" , "Bird that sang "Willow ____"" , "One of various small common birds" , "Type of bird, may be blue" , "another bird gets both" .)