The friend dad left behind (3)
I believe the answer is:
pal
'the friend' is the definition.
(pal is a kind of friend)
'dad left behind' is the wordplay.
'dad' becomes 'pa' ('pa' can be a synonym of 'dad').
'left' becomes 'l' (common abbreviation).
'behind' says to put 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.)" .)