Captain Caper has the French nonmetrical hymns (9)
I believe the answer is:
canticles
'hymns' is the definition.
(canticle is a kind of hymn)
'captain caper has the french' is the wordplay.
'captain' becomes 'c' (this might be a standard abbreviation of which I'm unaware).
'caper' becomes 'antics'.
'has' indicates putting letters inside.
'the french' becomes 'le' ('the' in French).
'antics' placed around 'le' is 'anticles'.
'c'+'anticles'='CANTICLES'
'nonmetrical' is the link.
I am not very happy about this link. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for canticles that I've seen before include "Church hymns" , "Songs used in church" .)