Fiddlestick that is used by rock star (5)
I believe the answer is:
bowie
'rock star' is the definition.
(David Bowie)
'fiddlestick that is' is the wordplay.
'fiddlestick' becomes 'bow'.
'that is' becomes 'ie' ('i.e.' can be a synonym of 'that is').
'bow'+'ie'='BOWIE'
'used by' acts as a link.
(Other definitions for bowie that I've seen before include "Rock-based chameleon" , "David --, singer" , "Dagger-knife" , "American frontiersman, killed at the Alamo, 1836" , "David --, rock singer/songwriter" .)