Let down, scowl, losing head (5)
I believe the answer is:
lower
'let down' is the definition.
(I know that let down can be written as lower)
'scowl losing head' is the wordplay.
'scowl' becomes 'glower' (I've seen this in another clue).
'losing head' means to remove the first letter.
'glower' with its first letter taken off is 'LOWER'.
(Other definitions for lower that I've seen before include "Let down using rowel" , "Depress" , "Let down, being inferior" , "Bring down" , "Hereford, say" .)