Convince Salvation Army to turn up? Certainly (6)
I believe the answer is:
assure
'convince' is the definition.
('assure' can be a synonym of 'convince')
'salvation army to turn up? certainly' is the wordplay.
'salvation army' becomes 'sa' (abbreviation).
'to turn up?' is a reversal indicator (I've seen 'turn up' mean this).
'certainly' becomes 'sure' ('sure' can be a synonym of 'certainly').
'sa' reversed gives 'as'.
'as'+'sure'='ASSURE'
(Other definitions for assure that I've seen before include "Cover with a policy" , "Tell positively" , "Say positively" , "Give confidence to" , "Impart confidence" .)