… one ensuring shades stay put - sounding portentous (4)
I believe the answer is:
dyer
'one ensuring shades stay' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'sounding portentous' is the wordplay.
'sounding' indicates a 'sounds like' (homophone) clue.
'portentous' becomes 'dire' (I've seen this before).
'dire' sounds like 'DYER'.
'put' is the link.
Can you help me to learn more?
(Other definitions for dyer that I've seen before include "Person who colours cloth" , "One changing colour" , "Colourer of fabrics" , "Increase (in output)" , "Cloth-stainer" .)