The way to dishearten a Roundhead (4)
I believe the answer is:
road
'the way' is the definition.
(road is a kind of way)
'dishearten a roundhead' is the wordplay.
'dishearten' means to remove the middle letters.
'roundhead' with its centre taken out is 'ROAD'.
'to' is the link.
(Other definitions for road that I've seen before include "See 2" , "Way to achieving a particular outcome" , "Abbey -, Beatles album" , "mine gallery" , "'The . . . . to hell is paved with good intentions'" .)