Wild rodent that is outside (5)
I believe the answer is:
irate
'wild' is the definition.
(I've seen this before)
'rodent that is outside' is the wordplay.
'rodent' becomes 'rat' (rat is a kind of rodent).
'that is' becomes 'ie' (abbreviation for id est, 'that is' in Latin).
'outside' means one lot of letters goes inside another (some letters go outside others).
'rat' inserted inside 'ie' is 'IRATE'.
(Other definitions for irate that I've seen before include "become irritated" , "A tier (anag)" , "Hacked off" , "Angry, annoyed" , "Infuriated" .)