River reaching middle of Caspian Sea (4)
I believe the answer is:
deep
'sea' is the definition.
(thesaurus)
'river reaching middle of caspian' is the wordplay.
'river' becomes 'dee' (River Dee**).
'reaching' says to put letters next to each other.
'middle of' says to take the centre.
The middle of 'caspian' is 'p'.
'dee'+'p'='DEEP'
(Other definitions for deep that I've seen before include "Intense -- extreme" , "late" , "Wise - difficult to understand" , "Main" , "Mysterious" .)