One class is angry (5)
I believe the answer is:
irate
'angry' is the definition.
(irate can mean angry or annoyed)
'one class' is the wordplay.
'one' becomes 'i' (Roman numeral).
'class' becomes 'rate' (class can mean to rank or rate).
'i'+'rate'='IRATE'
'is' is the link.
(Other definitions for irate that I've seen before include "Angry and incensed" , "Annoyed, exasperated" , "raging" , "Angry, enraged" , "will be up in arms" .)