Type of forest in Iran? (4)

I believe the answer is:

rain

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

'type of forest' is the definition.
(I've seen this in another clue)

'iran?' is the wordplay.
'iran' anagrams to 'RAIN'.
However, I am unsure how this is indicated.

'in' is the link.

Can you help me to learn more?

(Other definitions for rain that I've seen before include "Falling water" , "Fall in large quantities" , "Wet weather" , "downfall?" , "Storm" .)

I've seen this clue in the Evening Standard.

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