Magnificent piece of old Manx cat (4)
I believe the answer is:
opus
'magnificent piece' is the definition.
(as in a magnum opus)
'old manx cat' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'manx' means to remove the last letter (Manx cats have no tail).
'cat' becomes 'puss' (puss is a kind of cat).
'puss' with its final letter taken away is 'pus'.
'o'+'pus'='OPUS'
'of' is the link.
(Other definitions for opus that I've seen before include "Artistic work" , "Major work" , "Musical piece" , "(Numbered) musical composition" , "Composition - magnum __" .)