Master, extremely disagreeable in a gym (5)
I believe the answer is:
adept
'master' is the definition.
(I've seen this before)
'extremely disagreeable in a gym' is the wordplay.
'extremely' suggests removing the centre (only the extremes of the word are used).
'in' is an insertion indicator.
'gym' becomes 'PT' (Physical Training).
'disagreeable' with its centre taken out is 'de'.
'a'+'pt'='apt'
'de' going into 'apt' is 'ADEPT'.
(Other definitions for adept that I've seen before include "Taped (anag.)" , "who gets the lapis philosophicus secret" , "One very skilled at work" , "Deacon" , "One highly skilled could be taped" .)