Become disillusioned (4,5)
I believe the answer is:
lose heart
'become disillusioned' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for lose heart that I've seen before include "Become disillusioned" , "Become discouraged in cardiac manner?" , "Become despondent" , "Here's a lot to cause courage to ebb" , "Start to despair" .)