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 certain of 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 can't justify this - if you can you should give a lot more credence to this answer).
'en' put 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" .)