I'm surprised by fellow doing spells in resistance (6)

I believe the answer is:

ohmage

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'resistance' is the definition.
(ohmage is a kind of resistance)

'i'm surprised by fellow doing spells' is the wordplay.
'i'm surprised' becomes 'oh' (expression of surprise).
'by fellow doing spells' becomes 'mage' (I can't justify this - if you can you should give a lot more credence to this answer).
'oh'+'mage'='OHMAGE'

'in' acts as a link.

Can you help me to learn more?

(Another definition for ohmage that I've seen is " measure of resistance".)

I've seen this clue in The Times.

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