In Clare never ending contest for musical instrument (7)
I believe the answer is:
clavier
'musical instrument' is the definition.
(clavier is a kind of musical instrument)
'in clare never ending contest' is the wordplay.
'in' means one lot of letters goes inside another.
'never ending' means to remove the last letter.
'contest' becomes 'vie' (I've seen this before).
'clare' with its final letter taken off is 'clar'.
'clar' placed around 'vie' is 'CLAVIER'.
'for' is the link.
(Other definitions for clavier that I've seen before include "Instrumental keyboard" , "Stringed instrument with a keyboard" , "Bach played it" .)