Put off a letter to a relative (5)
I believe the answer is:
daunt
'put off' is the definition.
(I've seen this in another clue)
'a letter to a relative' is the wordplay.
'a letter' becomes 'd' ().
'to a relative' becomes 'aunt' (aunt is a type of relative).
'd'+'aunt'='DAUNT'
(Other definitions for daunt that I've seen before include "Cow" , "Cause to feel intimidated" , "Scare off - dismay" , "Dishearten" , "Frighten, intimidate" .)