Condemn old partner — gosh, a Greek troublemaker! (9)
I believe the answer is:
excoriate
'condemn' is the definition.
(synonyms)
'old partner gosh a greek troublemaker' is the wordplay.
'old partner' becomes 'ex' (an ex-partner).
'gosh' becomes 'cor' (I've seen this before).
'a greek' becomes 'i' (I can't explain this - if you can you should believe this answer much more).
'troublemaker' becomes 'ate' (Greek goddess of mischief).
'ex'+'cor'+'i'+'ate'='EXCORIATE'
Can you help me to learn more?
(Other definitions for excoriate that I've seen before include "roast" , "Flay" .)