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 don't see how they can define each other.
'it seems is a rodent about to gnaw his bib' is the wordplay.
I cannot quite see 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 within the answer.
A single letter 'r' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
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" .)