Friend of dad, fifty (3)
I believe the answer is:
pal
'friend' is the definition.
(pal can mean a friend)
'dad fifty' is the wordplay.
'dad' becomes 'pa' (both can mean one's father).
'fifty' becomes 'L' (Roman numeral).
'pa'+'l'='PAL'
'of' acts as a link.
(Other definitions for pal that I've seen before include "Close friend" , "China" , "Mate, buddy" , "Brother" , "Mate, informal friend" .)