Commonplace letter to write, it's said (5)
I believe the answer is:
trite
'commonplace' is the definition.
(I know that commonplace can be written as trite)
'letter to write it's said' is the wordplay.
'letter' becomes 't' ().
'to' means one lot of letters go next to another.
'it's said' shows a homophone (sound like).
'write' sounds like 'rite'.
't'+'rite'='TRITE'
(Other definitions for trite that I've seen before include "(Of an idea) lacking in originality" , "Banal, commonplace" , "Lyre string" , "repeated too often" , "Overfamiliar through overuse" .)