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' shows that the letters should be reversed in order.
'under' says to put letters next to each other (below, in a down clue).
'arch' becomes 'coy' (I've seen this in other clues).
'up' backwards is '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" .)