… 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' shows a homophone (sound like).
'portentous' becomes 'dire' (I've seen this before).
'dire' is a homophone of 'DYER'.
'put' acts as a 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" .)