Once having ordered one back inside, proceed (8)
I believe the answer is:
continue
'proceed' is the definition.
(synonyms)
'once having ordered one back inside' is the wordplay.
'having ordered' indicates an anagram.
'one' becomes 'unit' (synonyms).
'back' is a reversal indicator.
'inside' indicates putting letters inside.
'unit' back-to-front is 'tinu'.
'once' anagrammed gives '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" .)