Rhythm builds regularly during exercises (5)
I believe the answer is:
'rhythm' is the definition.
(I have seen 'Regular rhythm ' mean 'pulse' so perhaps 'rhythm' could also mean 'pulse')
'builds regularly during exercises' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'during' indicates putting letters inside.
'exercises' becomes 'pe' (Physical Education class).
The alternate letters of 'builds' are 'uls'.
'uls' inserted within 'pe' is 'PULSE'.
(Other definitions for pulse that I've seen before include "Vitality" , "Beat or throb of heart" , "Lentil, bean, seeds" , "often sought medically?" , "Beat - legumes" .)