They look mousy in a big way (4)
I believe the answer is:
rats
'they' is the definition.
Both the answer and definition are plural nouns.
Maybe you can see an association between them that I don't see?
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for rats that I've seen before include "Heels" , "First word of 'The 21 19'" , "Treacherous rodents" , "Damn" , "Rodent pests" .)