Beat back section of Royal Fusiliers (4)
I believe the answer is:
flay
'beat' is the definition.
(flay can mean to beat)
'back section of royal fusiliers' is the wordplay.
'back' says the letters should be written backwards.
'section of' indicates the answer is hidden within the clue.
'yalf' is hidden within 'royal fusiliers'.
'yalf' back-to-front is 'FLAY'.
(Other definitions for flay that I've seen before include "Fiercely criticise" , "Strip skin off by whipping" , "Skin alive" , "Strip skin from (a body)" , "Strip the skin from (a carcass)" .)