Mouldy? Has to end in decay (5)
I believe the answer is:
musty
'mouldy?' is the definition.
(I know that mouldy can be written as musty)
'has to end in decay' is the wordplay.
'has to' becomes 'must' (I've seen this before).
'end in' indicates one should take the final letters.
The final letter of 'decay' is 'y'.
'must'+'y'='MUSTY'
(Other definitions for musty that I've seen before include "Mouldy; outdated" , "Smelling of mould" , "Stale and dank" , "Stale and stuffy" , "Frowsty" .)