A fathead needs directions, if not many (3)
I believe the answer is:
few
'not many' is the definition.
('few' is the opposite to 'many')
'a fathead needs directions' is the wordplay.
'a fathead' becomes 'f' (head letter of 'fat').
'needs' is a charade indicator (letters next to each other).
'directions' becomes 'ew' (abbreviations for east and west).
'f'+'ew'='FEW'
'if' is the link.
(Other definitions for few that I've seen before include "'''Many are called but ... are chosen'' (Bible) (3)'" , "Only one or two" , "Indefinite but small number" , "Relatively small number" , "Not many, just a handful" .)