Leading some back into the garrison (8)
I believe the answer is:
foremost
'leading' is the definition.
(foremost means leading or principal)
'some back into the garrison' is the wordplay.
'back' says the letters should be written in reverse.
'into' is an insertion indicator.
'the garrison' becomes 'fort' (synonyms).
'some' written backwards gives 'emos'.
'emos' going inside 'fort' is 'FOREMOST'.
(Other definitions for foremost that I've seen before include "First in rank" , "top" , "Pre-eminent" , "'Leading, best (8)'" , "premier" .)