Sap will be encouraged (4,5)
I believe the answer is:
take heart
'encouraged' is the definition.
(I've seen this before)
'sap will be' is the wordplay.
I cannot quite see how this works, but
'will' could be 'heart' (both can mean desire or drive) and 'heart' is found within the answer.
'be' could be 'take' (taking is a kind of being) and 'take' is found in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for take heart that I've seen before include "Be encouraged or cheered up in cardiac way" , "Don't be discouraged - seek cardiac support (4,5)" , "Have courage, acquire cardiac boost" , "to leave outside leaves of lettuce" , "how to gain courage" .)