They look mousy in a big way (4)
I believe the answer is:
rats
'they' is the definition.
Both the definition and answer are plural nouns.
Maybe there's a link between them I don't understand?
I cannot understand how the remainder of the clue works.
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" .)