Orders at intervals given top-priority back up (9)
I believe the answer is:
resurgent
'back up' is the definition.
(I've seen this before)
'orders at intervals given top-priority' is the wordplay.
'at intervals' means one should take alternating letters (letters are taken at regular intervals).
'given' says to put letters next to each other.
'top-priority' becomes 'urgent' (I've seen this in another clue).
The alternate letters of 'orders' are 'res'.
'res'+'urgent'='RESURGENT'
(Other definitions for resurgent that I've seen before include "Reviving in popularity" , "Reviving after period of inactivity" , "Reviving after a lull" , "reborn" , "given new life" .)