First peer, this writer estimated (8)
I believe the answer is:
earliest
'first' is the definition.
(similar in meaning)
'peer this writer estimated' is the wordplay.
'peer' becomes 'earl' (earl is a kind of peer).
'this writer' becomes 'I' (the person who is writing these clues).
'estimated' becomes 'est' (abbreviation).
'earl'+'i'+'est'='EARLIEST'
(Other definitions for earliest that I've seen before include "The first to arrive - to get the worm?" , "First occurring" , "Arriving first" , "Next available" , "Furthest back in time" .)