Substitute for axes (5)
I believe the answer is:
proxy
'substitute' is the definition.
(I've seen this before)
'for axes' is the wordplay.
'for' becomes 'pro' (as in being 'pro' something).
'axes' becomes 'xy' (x axis and y axis).
'pro'+'xy'='PROXY'
(Other definitions for proxy that I've seen before include "One voting on behalf of another" , "Authorised agent" , "Authorised substitute" , "Stand in" , "Person acting for another" .)