Tease lad, terribly coarse (6)
I believe the answer is:
ribald
'coarse' is the definition.
(I've seen this in another clue)
'tease lad terribly' is the wordplay.
'tease' becomes 'rib' (I've seen this before).
'terribly' indicates anagramming the letters.
'lad' is an anagram of 'ald'.
'rib'+'ald'='RIBALD'
(Other definitions for ribald that I've seen before include "Irreverent" , "Licentious" , "Coarsely mocking" , "Indecent" , "Coarsely humorous; scurrilous" .)