Servant: need you back! (6)
I believe the answer is:
lackey
'servant' is the definition.
(a lackey is a servant or assistant)
'need you back' is the wordplay.
'need' becomes 'lack' (to lack something is to be in need of it).
'you' becomes 'ye' (historical version of 'you').
'back' says the letters should be written backwards.
'ye' backwards is 'ey'.
'lack'+'ey'='LACKEY'
(Other definitions for lackey that I've seen before include "assistant" , "Sycophant, toady" , "A servile follower or old servant" , "An archaic menial or follower" , "Obsequious follower" .)