Quick frolic in the middle of physical exercise (6)
I believe the answer is:
prompt
'quick' is the definition.
(I know that quick can be written as prompt)
'frolic in the middle of physical exercise' is the wordplay.
'frolic' becomes 'romp' (synonyms).
'in the middle of' is an insertion indicator.
'physical exercise' becomes 'PT' (Physical Training).
'romp' going inside 'pt' is 'PROMPT'.
(Other definitions for prompt that I've seen before include "With little or no delay" , "Cue given to actor" , "Precisely on time" , "On the dot" , "Immediate - supply forgotten words" .)