Plain above head of Tiber (5)
I believe the answer is:
overt
'plain' is the definition.
(both can mean visible or obvious)
'above head of tiber' is the wordplay.
'above' becomes 'over' (I've seen this before).
'head of' suggests taking the first letters.
The initial letter of 'tiber' is 't'.
'over'+'t'='OVERT'
(Other definitions for overt that I've seen before include "Open, up front" , "Conspicuous" , "Open and undisguised" , "Done or shown openly" , "Voter (anag.)" .)