Piece of, say, pieces in stone (7)
I believe the answer is:
segment
'piece' is the definition.
(I know that segment is a type of piece)
'say pieces in stone' is the wordplay.
'say' becomes 'eg' (both can mean for example).
'pieces' becomes 'men' (e.g. in chess).
'in' is an insertion indicator.
'stone' becomes 'st' (abbreviation).
'eg'+'men'='egmen'
'egmen' inserted inside 'st' is 'SEGMENT'.
'of' acts as a link.
(Other definitions for segment that I've seen before include "(Divided-up) part" , "Piece of an orange" , "Part, piece" , "split up" , "Division" .)