One used by gunner or forest official to tackle dreadful fiend (5-6)

I believe the answer is:

range-finder

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'one used by gunner' is the definition.
I know nothing about this answer so I can't tell whether this works.

'forest official to tackle dreadful fiend' is the wordplay.
'forest official' becomes 'ranger' (I've seen this in another clue).
'to tackle' is an insertion indicator (tackle can mean to take hold of).
'dreadful' indicates an anagram.
'fiend' anagrammed gives 'finde'.
'ranger' going around 'finde' is 'RANGE-FINDER'.

'or' acts as a link.

Can you help me to learn more?

I've seen this clue in The Hindu.

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