Sleep in regularly? Certainly, for relaxation (7)
I believe the answer is:
leisure
'for relaxation' is the definition.
(leisure is a kind of relaxation)
'sleep in regularly? certainly' is the wordplay.
'regularly?' indicates alternate letters (regularly take one letter, leave next etc.).
'certainly' becomes 'sure' (synonyms).
'sleep'+'in'='sleepin'
The alternating letters of 'sleepin' are 'lei'.
'lei'+'sure'='LEISURE'
(Other definitions for leisure that I've seen before include "Breathing space" , "Time when not working" , "Spare time" , "State of rest or repose" , "Time for relaxing" .)