A letter I composed is well expressed (8)
I believe the answer is:
literate
'well expressed' is the definition.
(I've seen this before)
'a letter i composed' is the wordplay.
'composed' indicates anagramming the letters.
'a'+'letter'+'i'='aletteri'
'aletteri' with letters rearranged gives 'LITERATE'.
'is' is the link.
(Other definitions for literate that I've seen before include "Relate it, being well read" , "Scholarly" , "Well versed in books" , "well expressed" , "version of 'speed of light'?" .)