Setter's almost broken foot (6)
I believe the answer is:
iambus
'foot' is the definition.
(iambus is a kind of foot)
'setter's almost broken' is the wordplay.
'setter's' becomes 'I am' (to the crossword setter, 'setter is' means 'I am').
'almost broken' becomes 'bus' (most of the word 'bust').
'iam'+'bus'='IAMBUS'
(Other definitions for iambus that I've seen before include "Metrical unit of two syllables" , "Type of metrical unit in a poem" , "poetic form" , "Metrical foot" .)