Dream of black sheep returning home at last (9)
I believe the answer is:
nightmare
'dream' is the definition.
(I've seen this before)
'black sheep returning home at last' is the wordplay.
'black' becomes 'night' (I've seen this in other clues**).
'sheep' becomes 'ram' (ram is a kind of sheep).
'returning' says the letters should be written backwards.
'at last' indicates one should take the final letters.
The final letter of 'home' is 'e'.
'ram' written backwards gives 'mar'.
'night'+'mar'+'e'='NIGHTMARE'
'of' acts as a link.
(Other definitions for nightmare that I've seen before include "that might upset the rest" , "Awful experience" , "Frightening dream" , "It makes one restless" , "causing distress to the Lord Chancellor?" .)