Commonplace letter to write, it's said (5)
I believe the answer is:
trite
'commonplace' is the definition.
(synonyms)
'letter to write it's said' is the wordplay.
'letter' becomes 't' ().
'to' says to put letters next to each other.
'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" .)