Small bird — cat finally got the thing (6)
I believe the answer is:
tomtit
'small bird' is the definition.
(I've seen this before)
'cat finally got the thing' is the wordplay.
'cat' becomes 'tom' (a tom is a male cat).
'finally' indicates one should take the final letters.
'the thing' becomes 'it'.
The final letter of 'got' is 't'.
'tom'+'t'+'it'='TOMTIT'
(Other definitions for tomtit that I've seen before include "One of various small common birds" , "bird - with 25d" , "Bird that sang "Willow ____"" , "Type of bird, may be blue" , "another bird gets both" .)