Muse tends to confuse — it's inevitable (5,4)
I believe the answer is:
needs must
'inevitable' is the definition.
(I've seen this before)
'muse tends to confuse' is the wordplay.
'to confuse' indicates an anagram.
'muse'+'tends'='musetends'
'musetends' is an anagram of 'NEEDS MUST'.
'it's' is the link.
(Other definitions for needs must that I've seen before include "It's inevitable" , "Of necessity" , "It is necessary" , "what wine-making does" , "Perforce" .)