PM's rant of one deranged (9)
I believe the answer is:
afternoon
'pm's' is the definition.
(I've seen this before)
'rant of one deranged' is the wordplay.
'deranged' indicates an anagram.
'rant'+'of'+'one'='rantofone'
'rantofone' anagrammed gives 'AFTERNOON'.
(Other definitions for afternoon that I've seen before include "Midday to evening" , "Time of day" , "several hours" , "PM includes this" , "Between noon and evening" .)