Deterioration of French island (5)
I believe the answer is:
decay
'deterioration' is the definition.
(I've seen this before)
'of french island' is the wordplay.
'of french' becomes 'de' ('of' in French).
'island' becomes 'cay' (I've seen this in another clue).
'de'+'cay'='DECAY'
(Other definitions for decay that I've seen before include "Rot, moulder" , "Rot, perish" , "Deterioration" , "Putrefy" , "Decline, decompose" .)