Without self-starters, gas must show full range (5)

I believe the answer is:

gamut

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'full range' is the definition.
(I've seen this in another clue)

I don't understand the remainder of the clue.

(Other definitions for gamut that I've seen before include "Full extent" , "The whole range of, say emotions" , "Complete range or scale" , "Entire scope" , "Complete range (of something)" .)

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

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