Unimaginative experts spies upset (7)
I believe the answer is:
prosaic
'unimaginative' is the definition.
(thesaurus)
'experts spies upset' is the wordplay.
'experts' becomes 'pros' (I've seen this in other clues).
'spies' becomes 'cia'.
'upset' says the letters should be written backwards.
'cia' reversed gives 'aic'.
'pros'+'aic'='PROSAIC'
(Other definitions for prosaic that I've seen before include "Lacking poetry" , "Ordinary - not poetical" , "Matter-of-fact" , "Dull (writing)" , "Commonplace, unromantic" .)