Angry as a rodent that is trapped (5)
I believe the answer is:
irate
'angry' is the definition.
(irate can mean angry or annoyed)
'as a rodent that is trapped' is the wordplay.
'as' becomes 'ie' (I can't justify this - if you can you should give a lot more credence to this answer).
'a rodent' becomes 'rat' (rat is a kind of rodent).
'that is trapped' is an insertion indicator.
'ie' placed around 'rat' is 'IRATE'.
Can you help me to learn more?
(Other definitions for irate that I've seen before include "Full of 22" , "Mad" , "Annoyed, irritated" , "Wrathful" , "Narked" .)