Radical half-trembles entering river (7)
I believe the answer is:
extreme
'radical' is the definition.
(I've seen this before)
'half-trembles entering river' is the wordplay.
'half' means to take half the letters.
'entering' is an insertion indicator.
'river' becomes 'exe' (River Exe).
'trembles' with half the letters taken is 'trem'.
'trem' placed into 'exe' is 'EXTREME'.
(Other definitions for extreme that I've seen before include "stiff" , "Farthest - to the highest degree" , "Point furthest away" , "Last" , "sheer" .)