Scowl when let down (5)
I believe the answer is:
lower
I believe this clue is a double definition.
'scowl' is the first definition.
(I've seen this before)
'let down' is the second definition.
(I know that let down can be written as lower)
'when' is the link.
(Other definitions for lower that I've seen before include "Depress" , "Bring down" , "Jersey perhaps" , "of lesser status" , "Move something down" .)