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'm not certain of the 'for' bit.
'a letter in duplication' is the wordplay.
'a letter' becomes 'en' (I've seen this in other clues).
'in' means one lot of letters goes inside another.
'duplication' becomes 'sse' (I can't justify this - if you can you should believe this answer much more).
'en' inserted within '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" .)