Stage exercises for drilling marines (7)
I believe the answer is:
perform
'stage' is the definition.
(I've seen this before)
'exercises for drilling marines' is the wordplay.
'exercises' becomes 'pe'.
'drilling' means one lot of letters goes inside another.
'marines' becomes 'rm' (abbreviation for 'Royal Marines').
'for' going into 'rm' is 'rform'.
'pe'+'rform'='PERFORM'
(Other definitions for perform that I've seen before include "Carry out; act" , "Carry out (action)" , "Act - accomplish" , "Act or execute" , "Do a good job" .)