The partner dad left (3)
I believe the answer is:
pal
'the partner' is the definition.
(I've seen this in other clues)
'dad left' is the wordplay.
'dad' becomes 'pa' (synonyms).
'left' becomes 'L' (common abbreviation).
'pa'+'l'='PAL'
(Other definitions for pal that I've seen before include "Buddy, chum" , "Mate, informal friend" , "Crony" , "Friend (colloq.)" , "China" .)