Be taken in by a letter to all in favour (4,3)
I believe the answer is:
fall for
'be taken in' is the definition.
(thesaurus)
'by a letter to all in favour' is the wordplay.
'by' is a charade indicator (letters next to each other).
'a letter' becomes 'f' ().
'to' is a charade indicator (letters next to each other).
'in favour' becomes 'for' (as in being for a particular cause).
'f'+'all'='fall'
'fall'+'for' is 'FALL FOR'.
(Other definitions for fall for that I've seen before include "Succumb to attraction, or be taken in" , "Take romantic liking to- steady on!" , "Be smitten by" , "Succumb to charms - take a tumble" , "Be taken in by - become infatuated with" .)