Involve volunteers in line-up (6)
I believe the answer is:
entail
'involve' is the definition.
(thesaurus)
'volunteers in line-up' is the wordplay.
'volunteers' becomes 'ta' (abbreviation for Territorial Army).
'in' indicates putting letters inside.
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'line' written backwards gives 'enil'.
'ta' placed inside 'enil' is 'ENTAIL'.
(Other definitions for entail that I've seen before include "Involve (something) as an inevitable consequence" , "Cause" , "Inalienable estate" , "Late in (anag.)" , "legal restriction" .)