Part of foot -- measure part of foot (6)
I believe the answer is:
instep
'part of foot' is the definition.
(I've seen this before)
'part of foot measure' is the wordplay.
'part of foot' becomes 'in' ('in' is part of 'foot').
'measure' becomes 'step' (step is a kind of measure**).
'in'+'step'='INSTEP'
(Other definitions for instep that I've seen before include "Arch area of the foot" , "Top of foot" , "Marching together correctly" , "One part of Oxford" , "Part of foot under spinet" .)