Function of heads of state in Near East (4)
I believe the answer is:
sine
'function' is the definition.
(mathematical function)
'heads of state in near east' is the wordplay.
'heads of' indicates taking the first letters.
'near' means one lot of letters go next to another.
'east' becomes 'e' ('E' can be a synonym of 'east').
The initial letter of 'state' is 's'.
's'+'in'+'e'='SINE'
'of' is the link.
(Other definitions for sine that I've seen before include "Indispensable condition" , "Latin tag for indefinitely - . . . . die" , "Trigonometric function" , "Without; maths function" , "Trig function" .)