Looked sullen when let down (7)
I believe the answer is:
lowered
I believe this is a double definition.
'looked sullen' is the first definition.
(I've seen this before)
'let down' is the second definition.
(I've seen this in another clue)
'when' acts as a link.
(Other definitions for lowered that I've seen before include "Let down gently" , "Moved downwards" , "Brought down" , "Moved something down" , "looked sullen" .)