Rodent’s thick, black coat reflected (3)
I believe the answer is:
rat
'rodent's' is the definition.
(rat is a kind of rodent)
'thick black coat reflected' is the wordplay.
'thick black coat' becomes 'tar' (thick, black substance).
'reflected' shows that the letters should be reversed in order (letters reflected back).
'tar' back-to-front is 'RAT'.
(Other definitions for rat that I've seen before include "one escaping doomed vessel?" , "shopper" , "Unpopular rodent" , "Unpleasant character" , "Desert - rodent" .)