A leader in Turkey supporting deception over line in pact (9)
I believe the answer is:
concordat
'pact' is the definition.
(thesaurus)
'a leader in turkey supporting deception over line' is the wordplay.
'leader in' suggests taking the first letters.
'supporting' is a charade indicator (letters next to each other) (in a down clue, some letters go below others).
'deception' becomes 'con' (I've seen this before).
'over' is a charade indicator (letters next to each other).
'line' becomes 'cord' (cord is a kind of line**).
The first letter of 'turkey' is 't'.
'a'+'t'='at'
'con'+'cord'='concord'
'at' put after 'concord' is 'CONCORDAT'.
'in' is the link.
(Other definitions for concordat that I've seen before include "Signed agreement" , "papal blessing" , "pact" , "Agreement with Rome" .)