Reason for having a letter in duplication (5)
I believe the answer is:
sense
'reason for' is the definition.
'sense' can be an answer for 'reason' (I've seen this in another clue). I am not sure about the 'for' bit.
'a letter in duplication' is the wordplay.
'a letter' becomes 'en' (I've seen this in other clues).
'in' is an insertion indicator.
'duplication' becomes 'sse' (I can't justify this - if you can you should believe this answer much more).
'en' put inside 'sse' is 'SENSE'.
'having' is the link.
Can you help me to learn more?
(Other definitions for sense that I've seen before include "Purport" , "Feel intuitively" , "Realistic attitude" , "One of five main faculties" , "Suspect" .)