Designate (for a purpose) (7)
I believe the answer is:
earmark
'designate for a purpose' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for earmark that I've seen before include "Allocate for a purpose" , "Reserve for later use" , "It identifies the outcome" , "Set aside for special purpose" , "Set aside for future use" .)