Musician — one deliberately getting the count wrong? (7)
I believe the answer is:
fiddler
This clue is a double definition.
'musician' is the first definition.
(fiddler can informally mean a violinist)
'one deliberately getting the count wrong?' is the second definition.
(someone fiddling with or manipulating a count)
(Other definitions for fiddler that I've seen before include "Violinist (informal)" , "Musician (on the roof)?" , "Violinist - swindler - type of crab" , "Type of crab; one on the Roof" , "foundd on the roof?" .)