Involve volunteers in overturning occupation (6)
I believe the answer is:
entail
'involve' is the definition.
(entail means to involve or require)
'volunteers in overturning occupation' is the wordplay.
'volunteers' becomes 'TA' (abbreviation for Territorial Army).
'in' means one lot of letters goes inside another.
'overturning' says the letters should be written backwards.
'occupation' becomes 'line' (synonyms).
'line' back-to-front is 'enil'.
'ta' inserted into 'enil' is 'ENTAIL'.
(Other definitions for entail that I've seen before include "Cut" , "Involve or require" , "Call for" , "Late in (anag.)" , "outmoded fashion" .)