Thin spies probing European pal (8)
I believe the answer is:
emaciate
'thin' is the definition.
(thesaurus)
'spies probing european pal' is the wordplay.
'spies' becomes 'CIA' (Central Intelligence Agency).
'probing' means one lot of letters goes inside another (probe can mean to make a hole in).
'european' becomes 'E' (abbreviation e.g. EU).
'pal' becomes 'mate' (I've seen this in other clues).
'e'+'mate'='emate'
'cia' placed into 'emate' is 'EMACIATE'.
(Other definitions for emaciate that I've seen before include "Waste away" , "get very thin" , "Make extremely thin" .)