Sketch includes river border (5)
I believe the answer is:
skirt
'border' is the definition.
(skirting is a kind of bordering)
'sketch includes river' is the wordplay.
'sketch' becomes 'skit' (I've seen this before).
'includes' means one lot of letters goes inside another.
'river' becomes 'r'.
'skit' going around 'r' is 'SKIRT'.
(Other definitions for skirt that I've seen before include "Item of female apparel" , "Garment below the waist" , "Go round edge" , "Go along the edge of" , "Ladies' garment" .)