It's smart to duplicate a letter in possibly any case (5)
I believe the answer is:
natty
'it's smart' is the definition.
(I've seen this before)
'in possibly any case' is the wordplay.
'in' is an insertion indicator.
'possibly' indicates an anagram.
'case' becomes 'tt' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'any' is an anagram of 'nay'.
'nay' going around 'tt' is 'NATTY'.
'to duplicate a letter' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for natty that I've seen before include "Stylishly dressed" , "Dapper (colloq.)" , "Smart, fashionable (colloq.)" , "Smartly dressed" , "Smart, spruce" .)