Rodent entering that is foaming at the mouth (5)
I believe the answer is:
irate
'foaming at the mouth' is the definition.
(foaming at the mouth means irate or angry)
'rodent entering that is' is the wordplay.
'rodent' becomes 'rat' (rat is a kind of rodent).
'entering' means one lot of letters goes inside another.
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'rat' placed into 'ie' is 'IRATE'.
(Other definitions for irate that I've seen before include "will be up in arms" , "ranting" , "Boiling" , "Aggrieved" , "Hacked off" .)