Measure on the inside of wing arch (6)
I believe the answer is:
instep
'arch' is the definition.
(instep is a kind of arch)
'measure on the inside of wing' is the wordplay.
'measure' becomes 'step' (synonyms).
'on' means one lot of letters go next to another.
'the inside of' says to take the centre.
The centre of 'wing' is 'in'.
'step' put after 'in' is 'INSTEP'.
(Other definitions for instep that I've seen before include "Sir Jack --, famous English cricketer" , "Set pin in part of foot" , "Part of foot under spinet" , "Part of foot or shoe" , "Arch area of the foot" .)