Peter, it seems, is a rodent about to gnaw his bib! (6)
I believe the answer is:
rabbit
'peter' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
'it seems is a rodent about to gnaw his bib' is the wordplay.
I cannot quite understand how this works, but
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is found in the answer.
'a' is found in the answer.
an anagram of 'bib' is 'bbi' which is present in the answer.
A single letter 'r' remains which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for rabbit that I've seen before include "Animal with hole" , "Weak player" , "Burrowing mammal" , "Bit bar (anag.)" , "Hare-like mammal" .)