Has to have a mould (4)
I believe the answer is:
must
I believe this clue is a triple definition!
'has to' is the first definition.
(I've seen this before)
'have' is the second definition.
('must' can be a synonym of 'have')
'a mould' is the third definition.
(I've seen this before)
(Other definitions for must that I've seen before include "Smell of dampness or staleness" , "The mould is compelled" , "Mouldiness" , "Essential to have" , "Prerequisite" .)