In one level is top-grade cat (5)
I believe the answer is:
tiger
'cat' is the definition.
(I know that tiger is a type of cat)
'in one level is top-grade' is the wordplay.
'in' is an insertion indicator.
'one level' becomes 'tier' (synonyms).
'is top' indicates taking the first letters (in a down clue, the first letter is top).
The initial letter of 'grade' is 'g'.
'tier' enclosing 'g' is 'TIGER'.
(Other definitions for tiger that I've seen before include "he got his stripes" , "Big cat of Asia" , "Large striped cat" , "Striped animal poetically burning bright" , "See 13" .)