Evident across mouth of Thames (5)
I believe the answer is:
overt
'evident' is the definition.
(I've seen this before)
'across mouth of thames' is the wordplay.
'across' becomes 'over' (synonyms).
'mouth of' suggests taking the first letters (mouth can mean something's origin).
The initial letter of 'thames' is 't'.
'over'+'t'='OVERT'
(Other definitions for overt that I've seen before include "Trove (anag.)" , "Open,up front" , "Voter (anag.)" , "Open, not 1 across" , "Patent, evident" .)