Guard against black sheep eating a rook (6)
I believe the answer is:
beware
'guard against' is the definition.
(I've seen this in another clue)
'black sheep eating a rook' is the wordplay.
'black' becomes 'b' (abbreviation as in pencil classifications).
'sheep' becomes 'ewe' (a ewe is a female sheep).
'eating' means one lot of letters goes inside another.
'rook' becomes 'R' (chess abbreviation).
'a'+'r'='ar'
'ewe' enclosing 'ar' is 'eware'.
'b'+'eware'='BEWARE'
(Other definitions for beware that I've seen before include "Steer clear of" , "Wee bra (anag)" , "Look out, take care" , "Archaic injunction to be careful, look out" , "See 18" .)