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' (short for Latin 'id est', 'that is').
'outside' is an insertion indicator (some letters go outside others).
'rat' put within 'ie' is 'IRATE'.
(Other definitions for irate that I've seen before include "become irritated" , "A tier (anag)" , "Hacked off" , "Angry, annoyed" , "Infuriated" .)