Compiler's commuting maybe? Not on foot (6)

I believe the answer is:

iambus

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'foot' is the definition.
(iambus is a kind of foot)

I don't understand how the remainder of the clue works.

(Other definitions for iambus that I've seen before include "Metrical unit of two syllables" , "Metrical foot" , "Type of metrical unit in a poem" , "poetic form" .)

I've seen this clue in The Independent.

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