Muse tends to wander - it's unavoidable (5,4)
I believe the answer is:
needs must
'unavoidable' is the definition.
(I've seen this before)
'muse tends to wander' is the wordplay.
'to wander' indicates anagramming the letters.
'muse'+'tends'='musetends'
'musetends' anagrammed gives 'NEEDS MUST'.
'it's' is the link.
(Other definitions for needs must that I've seen before include "'. . . . . . . . . when the devil drives'" , "there's no alternative" , "unavoidable" , "Perforce" , "It's inevitable" .)