Garland certain to get relaxation (7)
I believe the answer is:
leisure
'relaxation' is the definition.
(leisure is a kind of relaxation)
'garland certain' is the wordplay.
'garland' becomes 'lei' ('lei' can be a synonym of 'garland').
'certain' becomes 'sure' ('sure' can be a synonym of 'certain').
'lei'+'sure'='LEISURE'
'to get' is the link.
(Other definitions for leisure that I've seen before include "retirement" , "days off" , "Time when not working" , "Free time, comfort and convenience" , "Time for relaxation and pleasure" .)