Must die? (5)
I believe the answer is:
mould
This clue is a double definition.
'must' is the first definition.
(thesaurus)
'die?' is the second definition.
(I've seen this before)
(Other definitions for mould that I've seen before include "Shape" , "Must" , "development of earth" , "Penicillin was discovered in this" , "Growth due to fungi" .)