Certain to happen when Dicky invited duke to leave island retreat (10)
I believe the answer is:
inevitable
'certain to happen' is the definition.
(I've seen this before)
'dicky invited duke to leave island retreat' is the wordplay.
'dicky' indicates anagramming the letters (dicky can mean ill or non-functioning).
'duke' becomes 'd'.
'to leave' is a deletion indicator.
'island' becomes 'elba' (I've seen this before**).
'retreat' is a reversal indicator.
'invited' with 'd' removed is 'invite'.
'elba' back-to-front is 'able'.
'invite' is an anagram of 'inevit'.
'inevit'+'able'='INEVITABLE'
'when' is the link.
(Other definitions for inevitable that I've seen before include "Certain to happen" , "Inescapable" , "Unavoidable, fated" .)