Musician showing popular rocking talent is about to play guitar (15)
I believe the answer is:
instrumentalist
'musician' is the definition.
('instrumentalist' can be a synonym of 'musician')
'popular rocking talent is about to play guitar' is the subsidiary indication.
'popular' becomes 'in'.
'rocking' indicates an anagram.
'about' says to put letters next to each other.
'to play guitar' becomes 'strum' (I've seen this before).
'talent'+'is'='talentis'
'talentis' with letters rearranged gives 'entalist'.
'entalist' put after 'strum' is 'strumentalist'.
'in'+'strumentalist'='instrumentalist'
'showing' is the link.
(Other definitions for instrumentalist that I've seen before include "one of the band" , "oboe, perhaps" , "musician" .)