Initially indignant, rather angry, then enraged (5)
I believe the answer is:
irate
'enraged' is the definition.
(I've seen this in another clue)
'initially indignant rather angry' is the wordplay.
'initially' says to take the initial letters.
'rather angry' becomes 'rate' (I can't justify this - if you can you should believe this answer much more).
The first letter of 'indignant' is 'i'.
'i'+'rate'='IRATE'
'then' acts as a link.
Can you help me to learn more?
(Other definitions for irate that I've seen before include "Angry, enraged" , "Full of 22" , "will be up in arms" , "Angry and incensed" , "Cross, irritated" .)