Refrains from a useless letter (6)
I believe the answer is:
avoids
'refrains' is the definition.
(avoiding is a kind of refraining)
'a useless letter' is the wordplay.
'useless' becomes 'void'.
'letter' becomes 's'.
'a'+'void'+'s'='AVOIDS'
'from' is the link.
(Other definitions for avoids that I've seen before include "Keeps away or refrains from" , "Steers clear of, dodges" , "Escapes" , "Stays away from" , "Shuns, keeps away from" .)