Gentle reminder acceptable during exercise (4)
I believe the answer is:
poke
'gentle reminder' is the definition.
(poke can mean a reminder about something)
'acceptable during exercise' is the wordplay.
'acceptable' becomes 'ok' (OK can mean acceptable or fine).
'during' is an insertion indicator.
'exercise' becomes 'pe' (Physical Education).
'ok' placed inside 'pe' is 'POKE'.
(Other definitions for poke that I've seen before include "Ridicule - . . . . fun at" , "Probe with stick" , "Thrust forward; sack" , "Jab (with a finger)" , "Jab - old type of bonnet" .)