Fail to lower (3,4)
I believe the answer is:
let down
I believe this is a double definition.
'fail' is the first definition.
(thesaurus)
'lower' is the second definition.
(synonyms)
'to' is the link.
(Other definitions for let down that I've seen before include "Disappoint, fail to keep promise" , "Fail to meet expectations of others" , "deflating" , "Leave in the lurch" , "lengthened" .)