Time that is left over for certain? (7)
I believe the answer is:
leisure
'time' is the definition.
(I have seen 'Spare time' mean 'leisure' so perhaps 'time' could also mean 'leisure')
'that is left over for certain?' is the wordplay.
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'left' becomes 'l' (common abbreviation).
'over' shows that the letters should be reversed in order.
'for certain?' becomes 'sure' (synonyms).
'ie'+'l'='iel'
'iel' back-to-front is 'lei'.
'lei'+'sure'='LEISURE'
(Other definitions for leisure that I've seen before include "Period of relaxation" , "may suggest fun and games" , "Ease; time off work" , "Rue lies about rest period" , "casual wear" .)