Thoughtful letters read aloud (4)
I believe the answer is:
wise
'thoughtful' is the definition.
(thesaurus)
'letters read aloud' is the wordplay.
'letters' becomes 'Ys' (more than one Y).
'read aloud' shows a homophone (sound like).
'ys' sounds like 'WISE'.
(Other definitions for wise that I've seen before include "Morecambe partner" , "Sagacious" , "Sapient" , "Astute" , "Clever" .)