Hide prunes inside empty envelope (7)
I believe the answer is:
eclipse
'hide' is the definition.
(I know that eclipse is a more specific form of becloud)
'prunes inside empty envelope' is the wordplay.
'prunes' becomes 'clips' ('clip' can be a synonym of 'prune').
'inside' means one lot of letters goes inside another.
'empty' suggests removing the centre.
'envelope' with its centre taken out is 'ee'.
'clips' placed inside 'ee' is 'ECLIPSE'.
(Other definitions for eclipse that I've seen before include "dark?" , "Obscurement of one heavenly body by another" , "Black out" , "concealment" , "famous racehorse" .)