One supporter left, say (3)
I believe the answer is:
leg
'one supporter' is the definition.
'left say' is the wordplay.
'left' becomes 'l' (common abbreviation).
'say' becomes 'eg'.
'l'+'eg'='LEG'
(Other definitions for leg that I've seen before include "Run < on" , "prop" , "Support; cricket side" , "Section of journey; limb" , "Lower limb" .)