Pieces for one not completely secure (4)
I believe the answer is:
soli
'pieces for one' is the definition.
(more than one piece of music for a solo performer)
'not completely secure' is the wordplay.
'not completely' means to remove the last letter.
'secure' becomes 'solid' (similar in meaning).
'solid' with its last letter taken away is 'SOLI'.
(Other definitions for soli that I've seen before include "Parts for one performer" , "individual performances" , "one-man performances" .)