Fail to back up lease, depressed (3,4)
I believe the answer is:
let down
'fail to back up' is the definition.
(as in failing to back up a promise)
'lease depressed' is the wordplay.
'lease' becomes 'let' (synonyms).
'depressed' becomes 'down' (synonyms).
'let'+'down'='LET-DOWN'
(Other definitions for let down that I've seen before include "Aircraft descent" , "Fail to meet expectations of others" , "Disappoint, fail to keep promise" , "Failed - to deflate a tyre - disappointed" , "Failed - deflated" .)