Stop, not starting relaxation (4)
I believe the answer is:
ease
'relaxation' is the definition.
(synonyms)
'stop not starting' is the wordplay.
'stop' becomes 'cease' (synonyms).
'not starting' means to remove the first letter.
'cease' with its initial letter taken off is 'EASE'.
(Other definitions for ease that I've seen before include "Relieve or slacken" , "Freedom from worries" , "Facilitate; repose" , "At this, one is relaxed" , "Freedom from worry" .)