Green part in smoke that rolls over tongue (6)
I believe the answer is:
gaelic
'tongue' is the definition.
(Gaelic is an example)
'green part in smoke that rolls over' is the wordplay.
'green part' becomes 'lea' (a lea is a green area of land).
'in' indicates putting letters inside.
'smoke' becomes 'cig' (cigarette).
'that rolls over' is a reversal indicator (I've seen 'rolling over' mean this).
'lea' put inside 'cig' is 'cileag'.
'cileag' in reverse letter order is 'GAELIC'.
(Other definitions for gaelic that I've seen before include "Characteristic of the Celts" , "tongue" , "The native language of Ireland" , "Language of the Celts" , "Speech" .)