Sheep, black is a bit of a fast fighter? (6)
I believe the answer is:
ramjet
'a bit of a fast fighter?' is the definition.
(type of jet engine)
'sheep black' is the wordplay.
'sheep' becomes 'ram' (ram is a kind of sheep).
'black' becomes 'jet' ('jet' can be similar in meaning to 'black').
'ram'+'jet'='RAMJET'
'is' is the link.
(Other definitions for ramjet that I've seen before include "Type of aero-engine" , "Type of aircraft engine" , "Type of air engine" , "starter engine" .)