Mistake over blocking comedy sketch displaying root vegetable (7)
I believe the answer is:
skirret
'root vegetable' is the definition.
(type of vegetable)
'mistake over blocking comedy sketch' is the wordplay.
'mistake' becomes 'err' (synonyms).
'over' shows that the letters should be reversed in order.
'blocking' means one lot of letters goes inside another.
'comedy sketch' becomes 'skit' (a short comedy performance).
'err' in reverse letter order is 'rre'.
'rre' going within 'skit' is 'SKIRRET'.
'displaying' is the link.
(Other definitions for skirret that I've seen before include "tubers" , "Plant of parsley family with edible root; striker (anag.)" .)