Letters, say, soothe (4)
I believe the answer is:
ease
'soothe' is the definition.
(I know that ease is a more specific form of the action soothe)
'letters say' is the wordplay.
'letters' becomes 'es' (more than one E).
'say' shows a homophone (sound like).
'es' sounds like 'EASE'.
(Other definitions for ease that I've seen before include "Leisure, comfort" , "Calm" , "Rest and leisure" , "State of relaxation or rest" , "Loosen - relax" .)