Short letter sent, partially read? (5)
I believe the answer is:
terse
'short' is the definition.
('terse' can be a synonym of 'short')
'letter sent partially read?' is the wordplay.
'partially read?' indicates a hidden word (I've seen 'partially read' mean this).
'TERSE' can be found hidden inside 'letter sent'.
(Other definitions for terse that I've seen before include "Abrupt, concise" , "Reset to be pithy" , "Using few words" , "Of speech, short and pithy" , "Trees (anag.)" .)