Feel terribly sorry about gym's charge (6)
I believe the answer is:
repent
'feel terribly sorry about gym's charge' is the definition.
'repent' can be an answer for 'feel' (repenting is a kind of feeling). I am not certain of the 'terribly sorry about gym's charge' bit.
'about gym's charge' is the wordplay.
'about' indicates putting letters inside.
'gym' becomes 'pe' (short for Physical Education).
'charge' becomes 'rent' (I've seen this in other clues).
'pe' inserted within 'rent' is 'REPENT'.
Can you help me to learn more?
(Other definitions for repent that I've seen before include "Feel guilty" , "Feel or express contrition" , "Feel regret/remorse" , "Be contrite, ye sinners" , "Express remorse for sin" .)