Agrees to convert on the quiet, as forecast (7)
I believe the answer is:
presage
'forecast' is the definition.
(synonyms)
'agrees to convert on the quiet' is the wordplay.
'to convert' indicates anagramming the letters (I've seen 'converted' mean this).
'on' means one lot of letters goes inside another (as in clothing 'on' a person).
'the quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'agrees' with letters rearranged gives 'resage'.
'resage' placed around 'p' is 'PRESAGE'.
'as' is the link.
(Other definitions for presage that I've seen before include "Forecast, foreshadow" , "Foretell" , "Be a sign or warning of (an event)" , "Be a sign of (something)" , "Portent" .)