Cross that is worn by an informer (5)
I believe the answer is:
irate
'cross' is the definition.
(both can mean angry)
'that is worn by an informer' is the wordplay.
'that is' becomes 'ie' (abbreviation for id est, 'that is' in Latin).
'worn by' means one lot of letters goes inside another (outside letters are worn by those on the inside).
'an informer' becomes 'rat' ('rat' can be a synonym of 'informer').
'ie' placed around 'rat' is 'IRATE'.
(Other definitions for irate that I've seen before include "Annoyed, exasperated" , "Indignant, angry" , "ranting" , "Not happy" , "Angry and incensed" .)