The friend dad left behind (3)
I believe the answer is:
pal
'the friend' is the definition.
(pal can mean a friend)
'dad left behind' is the wordplay.
'dad' becomes 'pa' (both can mean one's father).
'left' becomes 'l' (common abbreviation).
'behind' is a charade indicator (letters next to each other).
'pa'+'l'='PAL'
(Other definitions for pal that I've seen before include "Mate, informal friend" , "Close friend" , "Buddy, chum" , "China" , "Friend (colloq.)" .)