Has to have mould - there's nothing else for it (5,4)
I believe the answer is:
needs must
'mould there's nothing else for it' is the definition.
I know nothing about this answer so I can't judge whether it can be defined by this definition.
'has to have' is the wordplay.
'has' becomes 'needs' ('need' can be a synonym of 'have').
'to have' becomes 'must' (synonyms).
'needs'+'must'='NEEDS MUST'
Can you help me to learn more?
(Other definitions for needs must that I've seen before include "unavoidable" , "'. . . . . . . . . when the devil drives'" , "what wine-making does" , "Of necessity" , "Perforce" .)