English river divided south of tidal mouth (5)
I believe the answer is:
trent
'english river' is the definition.
(River Trent in England)
'divided south of tidal mouth' is the wordplay.
'divided' becomes 'rent' (similar in meaning).
'south of' says to put letters next to each other.
'mouth' says to take the initial letters (mouth can mean something's entrance).
The initial letter of 'tidal' is 't'.
'rent' put after 't' is 'TRENT'.
(Other definitions for trent that I've seen before include "River that joins the Ouse to form the Humber" , "Eng. river; Council of -" , "Chief river of the Midlands" , "River through Newark" , "Little Nell family; Eng. river" .)