Fall in love and become dejected (4,5)
I believe the answer is:
lose heart
'fall in love' is the definition.
(I've seen this before)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for lose heart that I've seen before include "get discouraged?" , "What patient may do before subsequent transplant" , "Falter in courage - or fall in love? (4, 5)" , "Start to despair" , "Become despondent" .)