Left one on allotment (7)

I believe the answer is:

portion

'allotment' is the definition.
(portion is a kind of allotment)

'left one on' is the wordplay.
'left' becomes 'port' (left side of a ship - opposite of starboard).
'one' becomes 'i' (Roman numeral).
'port'+'i'+'on'='PORTION'

(Other definitions for portion that I've seen before include "Part - share" , "allotment" , "Segment" , "A lot of" , "Share, helping" .)

I've seen this clue in The Telegraph.

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