Father left as a mate (3)
I believe the answer is:
pal
'a mate' is the definition.
(I've seen this in another clue)
'father left' is the wordplay.
'father' becomes 'pa' (pa can mean dad or father).
'left' becomes 'l' (common abbreviation).
'pa'+'l'='PAL'
'as' acts as a link.
(Other definitions for pal that I've seen before include "Bud" , "Friend (colloq.)" , "Mate, buddy" , "Crony" , "Buddy, chum" .)