Ex-president supporting Democrat (4)
I believe the answer is:
ford
'ex-president' is the definition.
(I've seen this before)
'supporting democrat' is the wordplay.
'supporting' becomes 'for' (being for something means supporting it).
'democrat' becomes 'd'.
'for'+'d'='FORD'
(Other definitions for ford that I've seen before include "US President - river crossing" , "Shallow crossing point in river" , "White paint for the face" , "Wade across" , "Place to cross river" .)