It's not well done. Some extra rehearsal required (4)
I believe the answer is:
rare
'it's not well done' is the definition.
(I've seen this in another clue)
'some extra rehearsal required' is the wordplay.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'RARE' is hidden within 'extra rehearsal required'.
(Other definitions for rare that I've seen before include "Not often seen - underdone" , "Very lightly cooked, meat say" , "not dense" , "not often spotted" , "Scarce as hen's teeth!" .)