Furiously intense king is more menacing (7)

I believe the answer is:

fiercer

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

'more menacing' is the definition.
Both the answer and definition are comparative adjectives. Perhaps there's an association between them I don't understand?

'furiously intense king' is the wordplay.
I cannot really see how this works, but
'intense' could be 'fierce' (similar in meaning) and 'fierce' is present in the answer.
'king' could be 'r' (abbreviation for rex, king in Latin) and 'r' is present in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).

'is' is the link.

Can you help me to learn more?

(Other definitions for fiercer that I've seen before include "More savage" , "More intense" .)

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

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