Enquires, is tan treatment leak proof? (9)

I believe the answer is:

resistant

I'm a little stuck... Click here to teach me more about this clue!

'proof?' is the definition.
(I've seen this before)

'enquires is tan treatment leak' is the wordplay.
'RESISTANT' is hidden in amongst the letters.
However, I am uncertain how the hidden word is indicated.

Can you help me to learn more?

(Other definitions for resistant that I've seen before include "unlikely to be damaged?" , "Hindering the conductor" , "unaffected" , "Not affected by something, a bug by an antibiotic say" , "Impervious to various sources of damage" .)

I've seen this clue in the Sydney Morning Herald.

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