Musician initially performing in support (5)
I believe the answer is:
piper
'musician' is the definition.
(someone playing the bagpipes)
'initially performing in support' is the wordplay.
'initially' indicates taking the first letters.
'in' indicates putting letters inside.
'support' becomes 'pier' (type of structure).
The initial letter of 'performing' is 'p'.
'p' put into 'pier' is 'PIPER'.
(Other definitions for piper that I've seen before include "Scots musician" , "If you pay him, call the tune" , "Artist known for his stained glass in Coventry Cathedral, d. 1992" , "Bagpipes player" , "Musician sometimes paid (or pied!)" .)