Scathingly review article on Rex cat (7)
I believe the answer is:
panther
'cat' is the definition.
(I know that panther is a type of cat)
'scathingly review article on rex' is the wordplay.
'scathingly review' becomes 'pan' (to pan something is to give it a bad review).
'article' becomes 'the' (the definite article in English).
'on' means one lot of letters go next to another.
'rex' becomes 'R' (abbreviation e.g. GR=George Rex, King George).
'pan'+'the'+'r'='PANTHER'
(Other definitions for panther that I've seen before include "Big cat" , "animal!" , "kind of feline" , "Then rap the large black cat" , "Bagheera?" .)