Unkind to impair flu treatment (7)

I believe the answer is:

hurtful

'unkind' is the definition.
(similar in meaning)

'impair flu treatment' is the wordplay.
'impair' becomes 'hurt' (I've seen this in another clue).
'treatment' indicates an anagram (the letters must be processed or treated).
'flu' with letters rearranged gives 'ful'.
'hurt'+'ful'='HURTFUL'

'to' is the link.

(Another definition for hurtful that I've seen is " Upsetting, distressing".)

I've seen this clue in The Mirror.

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