Leading spies regressing, becoming obsolete (7)
I believe the answer is:
archaic
'obsolete' is the definition.
(I've seen this before)
'leading spies regressing' is the wordplay.
'leading' becomes 'arch' (e.g. in arch-enemy).
'spies' becomes 'CIA' (Central Intelligence Agency).
'regressing' shows that the letters should be reversed in order (to regress is to go backwards).
'cia' back-to-front is 'aic'.
'arch'+'aic'='ARCHAIC'
'becoming' acts as a link.
(Other definitions for archaic that I've seen before include "Of a much earlier period" , "No longer in general use" , "Belonging to former or ancient times" , "Antiquarian" , "Outdated" .)