Ancient chief backed CIA (7)
I believe the answer is:
archaic
'ancient' is the definition.
(I've seen this before)
'chief backed cia' is the wordplay.
'chief' becomes 'arch' (synonyms).
'backed' says the letters should be written in reverse.
'cia' in reverse letter order is 'aic'.
'arch'+'aic'='ARCHAIC'
(Other definitions for archaic that I've seen before include "From the past" , "Behind the times" , "Very old or old-fashioned" , "Outdated" , "Antiquated" .)