The partner dad left (3)
'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).
(Other definitions for pal that I've seen before include "Buddy, chum", "Mate, informal friend", "Crony", "Friend (colloq.)", "China".)