Sort of fire quick to gain ground? (5)

I believe the answer is:

rapid

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

'sort of fire quick' is the definition.
'rapid' can be an answer for 'quick' (thesaurus). I am not certain of the 'sort of fire' bit.

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for rapid that I've seen before include "Zippy" , "Speedy (5)" , "Flying" , "Express" , "moving fast" .)

I've seen this clue in the Evening Standard.

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