To read a letter means little (5)
I believe the answer is:
scant
'little' is the definition.
(I've seen this before)
'to read a letter' is the wordplay.
'to read' becomes 'scan' ('scan' can be a synonym of 'read').
'a letter' becomes 't' ().
'scan'+'t'='SCANT'
'means' is the link.
(Other definitions for scant that I've seen before include "Meagre" , "Hardly any" , "Barely sufficient" , "Barely adequate" , "Deficient, sparing" .)