Rodent turning up under arch (5)
I believe the answer is:
coypu
'rodent' is the definition.
(I know that coypu is a type of rodent)
'turning up under arch' is the wordplay.
'turning' says the letters should be written backwards.
'under' means one lot of letters go next to another (in a down clue, some letters go below others).
'arch' becomes 'coy' (arch can mean coy or mischievous).
'up' written backwards gives 'pu'.
'pu' put after 'coy' is 'COYPU'.
(Other definitions for coypu that I've seen before include "Beaver-like creature" , "aquatic mammal" , "South American rodent bred for its fur, nutria" , "Large South American beaver-like rodent" , "rodents" .)