It's taken when Reg, being angry, is out to get even (7)
I believe the answer is:
revenge
'get even' is the definition.
'revenge' can be an answer for 'get' (thesaurus). I'm not certain of the 'even' bit.
'it's taken when reg being angry is out' is the wordplay.
'it's taken when' becomes 'even' (I can't explain this - if you can you should give a lot more credence to this answer).
'being angry' indicates anagramming the letters.
'is out' means one lot of letters goes inside another.
'reg' with letters rearranged gives 'rge'.
'even' put inside 'rge' is 'REVENGE'.
'to' acts as a link.
Can you help me to learn more?
(Other definitions for revenge that I've seen before include "Malicious retaliation" , "Satisfaction taken for injury" , "Ship remembered in verse" , "Retribution" , "An eye for an eye" .)