Pardon warmonger periodically sent to pen (7)
I believe the answer is:
amnesty
'pardon' is the definition.
(I know that amnesty is a more specific form of pardon)
'warmonger periodically sent to pen' is the wordplay.
'periodically' means one should take alternating letters (letters must be selected 'every so often').
'sent to' means one lot of letters go next to another.
'pen' becomes 'sty' (both can mean an animal enclosure).
The alternate letters of 'warmonger' are 'amne'.
'amne'+'sty'='AMNESTY'
(Other definitions for amnesty that I've seen before include "Period of forgiveness for infractions" , "A general political pardon, perhaps for tax defaulters" , "Mass pardon of political kind" , "Organisation concerned with people unjustly imprisoned" , "General pardon - international charity" .)