Sketch encapsulates back of designer garment (5)
I believe the answer is:
skirt
'garment' is the definition.
(skirt is a kind of garment)
'sketch encapsulates back of designer' is the wordplay.
'sketch' becomes 'skit' (short comedy performance).
'encapsulates' is an insertion indicator.
'back of' indicates one should take the final letters.
The last letter of 'designer' is 'r'.
'skit' enclosing 'r' is 'SKIRT'.
(Other definitions for skirt that I've seen before include "Female garment" , "Fringe" , "Go along the edge of" , "Go round the edges of" , "Ladies' garment" .)