Setting exams is difficult (7)

I believe the answer is:

testing

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

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

'setting exams' is the wordplay.
'setting' anagrams to 'TESTING'.
Though, I am not clear how this is indicated.

'is' is the link.

Can you help me to learn more?

(Other definitions for testing that I've seen before include "of a difficult nature" , "Examining" , "... conducting exam" , "tiresome" , "Trying out" .)

I've seen this clue in The Mirror.

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