Firm at it trapping lake bird (4,3)
I believe the answer is:
coal tit
'bird' is the definition.
(I've seen this before)
'firm at it trapping lake' is the wordplay.
'firm' becomes 'co' (abbreviation for company).
'trapping' is an insertion indicator.
'lake' becomes 'L' (geographical abbreviation).
'at'+'it'='atit'
'atit' placed around 'l' is 'altit'.
'co'+'altit'='COAL-TIT'
(Other definitions for coal tit that I've seen before include "Little bird" , "Black-headed small European songbird" , "singer" , "Bird producing notes" , "Flyer" .)