Season when exercise in boxing arena curtailed (5)
I believe the answer is:
ripen
'season' is the definition.
(I've seen this before)
'exercise in boxing arena curtailed' is the wordplay.
'exercise' becomes 'pe' (Physical Education).
'in' is an insertion indicator.
'boxing arena' becomes 'ring' (boxing ring).
'curtailed' means to remove the last letter.
'ring' with its last letter taken off is 'rin'.
'pe' put within 'rin' is 'RIPEN'.
'when' is the link.
(Other definitions for ripen that I've seen before include "Become ready to eat" , "Become ready for picking" , "Come to maturity like fruit" , "Get ready for 2" , "Get ready to eat" .)