Base captured by spies, turning pale (7)
I believe the answer is:
anaemic
'pale' is the definition.
(I've seen this before)
'base captured by spies turning' is the wordplay.
'base' becomes 'mean' (synonyms).
'captured by' is an insertion indicator.
'spies' becomes 'CIA' (Central Intelligence Agency).
'turning' is a reversal indicator.
'mean' put within 'cia' is 'cimeana'.
'cimeana' backwards is 'ANAEMIC'.
(Other definitions for anaemic that I've seen before include "Lacking red blood cells" , "Lacking in red corpuscles" , "Pale, sickly" , "Suffering from a lack of red blood cells" , "strangely pale" .)