Glad that husband has left plenty of petrol (7)

I believe the answer is:

tankful

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'petrol' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for tankful that I've seen before include "load of fuel" , "Full load of petrol" .)

I've seen this clue in The Sunday Times.

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