Become discouraged (4,5)
I believe the answer is:
lose heart
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">become discouraged</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for lose heart that I've seen before include "Become discouraged in cardiac manner?" , "What patient may do before subsequent transplant" , "Let hopes dwindle - real ethos involved" , "find discouragement" , "Falter in courage - or fall in love? (4, 5)" .)