Compulsion to cleanse failing to start (4)
I believe the answer is:
urge
'compulsion' is the definition.
(I've seen this before)
'cleanse failing to start' is the wordplay.
'cleanse' becomes 'purge' (I've seen this before).
'failing to start' means to remove the first letter.
'purge' with its first letter taken off is 'URGE'.
'to' is the link.
(Other definitions for urge that I've seen before include "Sudden inclination to do something" , "Advocate" , "Longing" , "Press to do something" , "Induce; yearning" .)