Unconcealed on top of letter (5)
I believe the answer is:
overt
'unconcealed' is the definition.
(associated in meaning)
'on top of letter' is the wordplay.
'on top of' becomes 'over' (I've seen this before).
'letter' becomes 't' ().
'over'+'t'='OVERT'
(Other definitions for overt that I've seen before include "Trove (anag.)" , "Open, up front" , "This is not hard to see" , "Plain and obvious" , "Openly displayed" .)