Carry on once unit is given new order (8)
I believe the answer is:
continue
'carry on' is the definition.
('continue' can be a synonym of 'carry on')
'once unit is given new order' is the wordplay.
'is given' means one lot of letters go next to another.
'new order' is an anagram indicator.
'once'+'unit'='onceunit'
'onceunit' is an anagram of 'CONTINUE'.
(Other definitions for continue that I've seen before include "Go on without stopping" , "Proceed" , "Don't stop, keep going" , "Carry on" , "Persist" .)