Smelling mouldy? Does he have to tell? (5)
I believe the answer is:
musty
'smelling mouldy? does he' is the definition.
'musty' can be an answer for 'smelling' (I have seen 'Stale smelling ' mean 'musty' so perhaps 'smelling' could also mean 'musty'). I'm unsure of the 'mouldy? does he' bit.
'have to tell?' is the wordplay.
'have to' becomes 'must' (I've seen this before).
'tell?' becomes 'y' (I can't justify this - if you can you should give a lot more credence to this answer).
'must'+'y'='MUSTY'
Can you help me to learn more?
(Other definitions for musty that I've seen before include "Stale and stuffy" , "Frowsty" , "Smelling of mould" , "Stale, mouldy-smelling" , "Stale and dank" .)