The old man's a bit of a sharp operator! (3)
I believe the answer is:
pop
'the old man's' is the definition.
(both can mean one's father)
'a bit of a sharp operator' is the wordplay.
'a bit of' indicates the answer is hidden within the clue.
'POP' can be found hidden inside 'sharp operator'.
(Other definitions for pop that I've seen before include "Soft drink - father - music - light explosion" , "Modern music for the masses" , "Old jingle - . . . goes the weasel" , "Cartoon sailor (showing amazement?)" , "Music with mass appeal" .)