Involve the soldiers in the back row (6)
I believe the answer is:
entail
'involve' is the definition.
(thesaurus)
'the soldiers in the back row' is the wordplay.
'the soldiers' becomes 'ta' (abbreviation for Territorial Army).
'in the' is an insertion indicator.
'back' is a reversal indicator.
'row' becomes 'line' (I've seen this before).
'line' reversed gives 'enil'.
'ta' inserted into 'enil' is 'ENTAIL'.
(Other definitions for entail that I've seen before include "Cause" , "Bring on an inevitable consequence" , "Need" , "outmoded fashion" , "Call for" .)