Lower due to a disappointment? (3,4)
I believe the answer is:
let down
This clue is a double definition.
'lower' is the first definition.
(synonyms)
'a disappointment?' is the second definition.
(I've seen this before)
'due to' is the link.
(Other definitions for let down that I've seen before include "Disappointment < lowered" , "Fail to meet expectations of others" , "Disappoint, fail to keep promise" , "lengthened" , "Failed - deflated" .)