Rodent that is about to become angry (5)
I believe the answer is:
irate
'angry' is the definition.
(irate can mean angry or annoyed)
'rodent that is about' is the wordplay.
'rodent' becomes 'rat' (rat is a kind of rodent).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'about' means one lot of letters goes inside another.
'rat' put inside 'ie' is 'IRATE'.
'to become' is the link.
(Other definitions for irate that I've seen before include "raging" , "Livid" , "Annoyed, exasperated" , "Indignant, angry" , "Annoyed, irritated" .)