People in authority apprehending girl back in part of Greece (8)
I believe the answer is:
thessaly
'part of greece' is the definition.
('Thessaly' is part of 'greece')
'people in authority apprehending girl back' is the wordplay.
'people in authority' becomes 'they' (I've seen this in other clues).
'apprehending' indicates putting letters inside.
'girl' becomes 'lass' (lass is a kind of girl).
'back' shows that the letters should be reversed in order.
'lass' written backwards gives 'ssal'.
'they' placed around 'ssal' is 'THESSALY'.
'in' is the link.
(Other definitions for thessaly that I've seen before include "In Greece a plain" , "Region of central Greece" , "Aegean area" , "area of Greece" , "Greek region" .)