South American volunteers with popular support (7)
I believe the answer is:
sustain
'support' is the definition.
('sustain' can be a synonym of 'support')
'south american volunteers with popular' is the wordplay.
'south american' becomes 'sus' (I've seen this in other clues).
'volunteers' becomes 'ta' (abbreviation for Territorial Army).
'with' means one lot of letters go next to another.
'popular' becomes 'in'.
'sus'+'ta'+'in'='SUSTAIN'
(Other definitions for sustain that I've seen before include "Keep up; hold for a long time" , "Support, keep going" , "Undergo" , "Bear (something's weight)" , "Feed, nourish" .)