High-flyer seen in big gym, they say? (6)
I believe the answer is:
larkin
'big gym they say?' is the definition.
The definition suggests an adverb but the answer is not.
'high-flyer seen in' is the wordplay.
'high-flyer' becomes 'lark' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'seen' is a charade indicator (letters next to each other).
'lark'+'in'='LARKIN'
Can you help me to learn more?
(Other definitions for larkin that I've seen before include "English poet, d. 1985" , "Big Jim ..., trade unionist" , "Philip ---, C twenty English poet" , "Scheme" , "Hull librarian and poet, d. 1985" .)