Somewhat plump? Consuming gallons is justified (4-8)

I believe the answer is:

well-grounded

'is justified' is the definition.
(with good justification or grounds)

'somewhat plump? consuming gallons' is the wordplay.
'somewhat plump?' becomes 'wellrounded'.
'consuming' is an insertion indicator.
'gallons' becomes 'g' (abbreviation).
'wellrounded' enclosing 'g' is 'WELL-GROUNDED'.

I've seen this clue in The Times.

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