Greek city in the Adriatic or in Thessaly (7)
I believe the answer is:
corinth
'greek city' is the definition.
(I've seen this before)
'in the adriatic or in thessaly' is the wordplay.
'in' indicates the answer is hidden within the clue.
'CORINTH' can be found hidden inside 'adriatic or in thessaly'.
(Other definitions for corinth that I've seen before include "Region of ancient Greece" , "Greek Gulf, isthmus city" , "City of Greece" , "Ancient Greek city" , "Mediterranean port" .)