Once having ordered one back inside, proceed (8)
I believe the answer is:
continue
'proceed' is the definition.
('continue' can be a synonym of 'proceed')
'once having ordered one back inside' is the wordplay.
'having ordered' indicates anagramming the letters.
'one' becomes 'unit' ('unit' can be a synonym of 'one').
'back' shows that the letters should be reversed in order.
'inside' means one lot of letters goes inside another.
'unit' written backwards gives 'tinu'.
'once' is an anagram of 'cone'.
'cone' going around 'tinu' is 'CONTINUE'.
(Other definitions for continue that I've seen before include "Resume" , "Go on without stopping" , "Carry on" , "Proceed" , "Persist" .)