Strong leader for Greeks takes time to make mark (6)
I believe the answer is:
stigma
'mark' is the definition.
(I know that mark can be written as stigma)
'strong leader for greeks takes time' is the wordplay.
'strong leader for greeks' becomes 'sigma' (Greek equivalent of S, first letter of 'strong').
'takes' indicates putting letters inside.
'time' becomes 't' (abbreviation).
'sigma' enclosing 't' is 'STIGMA'.
'to make' is the link.
(Other definitions for stigma that I've seen before include "Mark of shame or social disgrace" , "Part of a flower" , "evidence of nastiness?" , "Blot" , "part of plant" .)