Personal assistant left an ally (3)
I believe the answer is:
pal
'an ally' is the definition.
(both can mean a friend or associate)
'personal assistant left' is the wordplay.
'personal assistant' becomes 'PA' (abbreviation).
'left' becomes 'l' (common abbreviation).
'pa'+'l'='PAL'
(Other definitions for pal that I've seen before include "Friend (colloq.)" , "Close friend" , "Crony" , "Mate, buddy" , "China" .)