Rustic couple on lake (5)
I believe the answer is:
yokel
'rustic' is the definition.
(yokel is a kind of rustic)
'couple on lake' is the wordplay.
'couple' becomes 'yoke' ('yoke' can be a synonym of 'couple'**).
'on' says to put letters next to each other.
'lake' becomes 'l'.
'yoke'+'l'='YOKEL'
(Other definitions for yokel that I've seen before include "Hick" , "Rustic person (derogatory)" , "Rustic - peasant" , "Country bumpkin" , "Unsophisticated rustic" .)