Over the top - that's clear (5)
I believe the answer is:
overt
'clear' is the definition.
(both can mean apparent or obvious)
'over the top' is the wordplay.
'over' means one lot of letters go next to another (in a down clue, some letters go over others).
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'top' becomes 'over' (I've seen this before).
't' after 'over' is 'OVERT'.
'that's' acts as a link.
(Other definitions for overt that I've seen before include "Openly displayed" , "Open and undisguised" , "Open, not 1 across" , "Explicit" , "Open and obvious" .)