Most trim little flier expires trapped inside (7)
I believe the answer is:
tidiest
'most trim' is the definition.
('tidy' can be similar in meaning to 'trim')
'little flier expires trapped inside' is the wordplay.
'little flier' becomes 'tit' (tit is a kind of birdanimal. I am not sure about the 'little' bit.**).
'expires' becomes 'dies' ('die' can be a synonym of 'expire').
'trapped inside' means one lot of letters goes inside another.
'tit' going around 'dies' is 'TIDIEST'.
Can you help me to learn more?
(Other definitions for tidiest that I've seen before include "Neatest" , "smarter than anything" , "most trim" , "One orderly more than others" , "optimally arranged" .)