Squatter — I must leave truck (6)

I believe the answer is:

dumper

'truck' is the definition.
(dumper is a kind of truck)

'squatter i must leave' is the wordplay.
'squatter' becomes 'dumpier' ('dumpy' can be a synonym of 'squat').
'must leave' suggests deleting specific letters.
'dumpier' with 'i' removed is 'DUMPER'.

(Another definition for dumper that I've seen is " Tilting truck".)

I've seen this clue in The Times.

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