Feel genuinely sorry about gym’s charge (6)
I believe the answer is:
repent
'feel genuinely sorry' is the definition.
'about gym's charge' is the wordplay.
'about' indicates putting letters inside.
'gym' becomes 'pe'.
'charge' becomes 'rent' (I've seen this in other clues).
'pe' inserted inside 'rent' is 'REPENT'.
(Other definitions for repent that I've seen before include "Express regret" , "Feel sincere regret" , "lying on the ground" , "Be contrite, ye sinners" , "Be sorry for wrongdoing" .)