Travelling in car, thunderous - if so, what's up? (4,5)

I believe the answer is:

rain cloud

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

'so what's up?' is the definition.
I can't judge whether this definition defines the answer.

'travelling in car thunderous' is the wordplay.
'travelling' indicates anagramming the letters.
'thunderous' becomes 'loud' (similar in meaning).
'in'+'car'='incar'
'incar' anagrammed gives 'rainc'.
'rainc'+'loud'='RAIN-CLOUD'

'if' acts as a link.

Can you help me to learn more?

(Other definitions for rain cloud that I've seen before include "Nimbus" , "Uncordial (anag.)" .)

I've seen this clue in The Guardian.

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