Muse tends to be bothered only when absolutely necessary (5,4)
I believe the answer is:
needs must
'bothered only when absolutely necessary' is the definition.
I don't know anything about this answer so I cannot tell whether it can be defined by this definition.
'muse tends to be' is the wordplay.
'to be' indicates anagramming the letters.
'muse'+'tends'='musetends'
'musetends' anagrammed gives 'NEEDS MUST'.
Can you help me to learn more?
(Other definitions for needs must that I've seen before include "Perforce" , "It's inevitable" , "'. . . . . . . . . when the devil drives'" , "It can't be helped" , "unavoidable" .)