Judge a rook by vicious dog? (7)
I believe the answer is:
arbiter
'judge' is the definition.
(I know that arbiter is a type of judge)
'a rook by vicious dog?' is the wordplay.
'rook' becomes 'R' (chess abbreviation).
'by' is a charade indicator (letters next to each other).
'vicious dog?' becomes 'biter' (I've seen this in another clue).
'a'+'r'+'biter'='ARBITER'
(Other definitions for arbiter that I've seen before include "Settler of disputes" , "expert" , "He'll decide" , "Judge, umpire" , "Rarebit? (anag)" .)