Expert, extremely derisive in a gym (5)
I believe the answer is:
adept
'expert' is the definition.
('adept' can be a synonym of 'expert')
'extremely derisive in a gym' is the wordplay.
'extremely' says to hollow out the word (remove centre letters).
'in' means one lot of letters goes inside another.
'gym' becomes 'pt'.
'derisive' with its centre removed is 'de'.
'a'+'pt'='apt'
'de' put into 'apt' is 'ADEPT'.
(Other definitions for adept that I've seen before include "Proficient, skilled" , "Skillful" , "Very skilful, proficient" , "One who is very skilful" , "Taped (anag.)" .)