Exercise with walks occasionally on top of a mountain (4)
I believe the answer is:
peak
'top of a mountain' is the definition.
(I've seen this in another clue)
'exercise with walks occasionally' is the wordplay.
'exercise' becomes 'pe' (Physical Education).
'with' is a charade indicator (letters next to each other).
'occasionally' means one should take alternating letters (letters taken at regular intervals or occasions).
The alternating letters of 'walks' are 'ak'.
'pe'+'ak'='PEAK'
'on' acts as a link.
(Other definitions for peak that I've seen before include "Reach the highest point" , "Acme" , "Pointed mountain" , "Tip" , "'Heyday'" .)