I spent out getting part of a shoe (6)
I believe the answer is:
'part of a shoe' is the definition.
('instep' is part of 'shoe')
'i spent out getting' is the wordplay.
'out' indicates anagramming the letters (out can mean wrong or inaccurate).
'getting' is a charade indicator (letters next to each other).
'spent' is an anagram of 'nstep'.
(Other definitions for instep that I've seen before include "Part of foot between toes and ankle" , "Foot arch - conforming" , "Arch area of the foot" , "Top of foot" , "Part of foot (how to march!)" .)