Fun unfortunately suffering for sure (9)

I believe the answer is:

unfailing

'sure' is the definition.
('unfailing' can be a synonym of 'sure')

'fun unfortunately suffering' is the wordplay.
'unfortunately' is an anagram indicator.
'suffering' becomes 'ailing' (ailing is a kind of suffering**).
'fun' with letters rearranged gives 'unf'.
'unf'+'ailing'='UNFAILING'

'for' is the link.

(Another definition for unfailing that I've seen is " Reliable, constant".)

I've seen this clue in The Times.

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