Sketch ending in drivel, speak slowly (5)
I believe the answer is:
drawl
'speak slowly' is the definition.
(I've seen this before)
'sketch ending in drivel' is the wordplay.
'sketch' becomes 'draw' (to sketch is to draw).
'ending in' says to take the final letters.
The last letter of 'drivel' is 'l'.
'draw'+'l'='DRAWL'
(Other definitions for drawl that I've seen before include "Speak languidly" , "Affected speech" , "Speak lazily with long vowel sounds" , "Jimmy Stewart spoke with one" , "Slow speech pattern" .)