Pieces for one not quite secure? (4)
I believe the answer is:
Here is my best explanation:
'pieces' is the definition.
(solo is a kind of piece)
'one not quite secure?' is the subsidiary indication.
'one not quite' means to remove the last letter.
'secure?' becomes 'solid' ('solid' can be similar in meaning to 'secure').
'solid' with its final letter removed is 'soli'.
'for' is the link.
(Other definitions for soli that I've seen before include "one-man performances", "Parts for one performer" and "individual performances".)