Musical composition means nothing to a Manx cat (4)
I believe the answer is:
opus
'musical composition' is the definition.
(synonyms)
'nothing to a manx cat' is the wordplay.
'nothing' becomes 'O' (looks like zero - 0).
'to a manx' means to remove the last letter (the word has no 'tail', like a Manx cat).
'cat' becomes 'puss' (puss is a kind of cat).
'puss' with its last letter taken off is 'pus'.
'o'+'pus'='OPUS'
'means' is the link.
(Other definitions for opus that I've seen before include "Artistic work" , "Creative work, eg in writing, art or music" , "Composition - magnum __" , "A Latin work" , "Work (mus.)" .)