A point behind? No good! (5)
I believe the answer is:
prong
'a point' is the definition.
(I've seen this before)
'behind? no good' is the wordplay.
'behind?' becomes 'pro' (to be pro something is to get behind it).
'no' becomes 'n' (common abbreviation).
'good' becomes 'g' (abbreviation).
'pro'+'n'+'g'='PRONG'
(Other definitions for prong that I've seen before include "Fork's point" , "Tooth" , "Something piercing" , "Pointed end of fork" , "Point of a fork" .)