Violin piece by leader in strong position (10)
I believe the answer is:
bridgehead
'strong position' is the definition.
(I've seen this before)
'violin piece by leader' is the wordplay.
'violin piece' becomes 'bridge'.
'by' means one lot of letters go next to another.
'leader' becomes 'head' (head is a kind of leader).
'bridge'+'head'='BRIDGEHEAD'
'in' is the link.
(Other definitions for bridgehead that I've seen before include "defensive position" , "Secured position from which to advance" , "Forward position seized by attacking troops" , "Strong position secured inside enemy territory" , "Advantageous position" .)