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' indicates putting letters inside.
'duplication' becomes 'sse' (I am not sure about this - if you are sure you should believe this answer much more).
'en' placed into 'sse' is 'SENSE'.
'having' acts as a 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" .)