PM's tepid memoirs rewritten (4,8)
I believe the answer is:
post meridiem
'pm's' is the definition.
('post meridiem' can be a synonym of 'pm')
'tepid memoirs rewritten' is the wordplay.
'rewritten' indicates an anagram.
'tepid'+'memoirs'='tepidmemoirs'
'tepidmemoirs' is an anagram of 'POST MERIDIEM'.
(Other definitions for post meridiem that I've seen before include "PM" , "Afternoon" , "After noon" .)