Cricketer’s heavy blow about exercise (6)
I believe the answer is:
opener
'cricketer's' is the definition.
(batsman who opens the batting)
'heavy blow about exercise' is the wordplay.
'heavy blow' becomes 'oner' (I've seen this in another clue).
'about' indicates putting letters inside.
'exercise' becomes 'pe' (Physical Education).
'oner' enclosing 'pe' is 'OPENER'.
(Other definitions for opener that I've seen before include "man, first to bat" , "No.1 or No.2 bat" , "Corkscrew, for example" , "player, first to bat" , "First goal in game" .)