Trying hard, not having succeeded in the gym? (8)
I believe the answer is:
training
'in the gym?' is the definition.
(someone might be training in the gym)
'trying hard not having succeeded' is the wordplay.
'trying hard' becomes 'straining' (straining one's body).
'not having' suggests deleting specific letters.
'succeeded' becomes 's' (genealogical abbreviation).
'straining' with 's' removed is 'TRAINING'.
(Other definitions for training that I've seen before include "The holding of an office" , "Exercising" , "Prepare for a competition or career" , "Vocational education" , "drill" .)