Informant inside that is angry (5)
I believe the answer is:
irate
'angry' is the definition.
('irate' can be similar in meaning to 'angry')
'informant inside that is' is the wordplay.
'informant' becomes 'rat' (rat is a kind of informant).
'inside' is an insertion indicator.
'that is' becomes 'ie' ('i.e.' can be a synonym of 'that is').
'rat' going into 'ie' is 'IRATE'.
(Other definitions for irate that I've seen before include "Narked" , "Angry, enraged" , "Aggrieved" , "Mad" , "Infuriated" .)