Part of shoe that's conforming (6)

I believe the answer is:

instep

I'm a little stuck... Click here to teach me more about this clue!

'part of shoe' is the definition.
('instep' is part of 'shoe')

I cannot understand how the rest of the clue works.

(Other definitions for instep that I've seen before include "Central part of a foot" , "Marching together correctly" , "Part of foot or shoe" , "Foot arch - conforming" , "Arched part of foot" .)

I've seen this clue in The Mirror.

Want a hint initially instead of a full solution? Install my app