Antisocial person left a certain party halfway through (5)
I believe the answer is:
loner
'antisocial person' is the definition.
(I've seen this before)
'left a certain party halfway through' is the wordplay.
'left' becomes 'l' (common abbreviation).
'a certain' becomes 'one' (I've seen this before).
'halfway through' indicates the central letters.
The middle of 'party' is 'r'.
'l'+'one'+'r'='LONER'
(Other definitions for loner that I've seen before include "Unsociable person" , "Individualist" , "One liking solitude" , "Person shunning company" , "No mixer" .)