The French island certainly supplies relaxation (7)
I believe the answer is:
leisure
'relaxation' is the definition.
(leisure is a kind of relaxation)
'the french island certainly' is the wordplay.
'the french' becomes 'le' ('the' in French).
'island' becomes 'i' (geographical abbreviation).
'certainly' becomes 'sure' (synonyms).
'le'+'i'+'sure'='LEISURE'
'supplies' acts as a link.
(Other definitions for leisure that I've seen before include "Ease; time off work" , "Recreation time" , "Freedom from work" , "Free time; relaxation" , "Use lire (anag)" .)