River Plate originally measuring many fathoms? (4)
I believe the answer is:
deep
'measuring many fathoms?' is the definition.
'river plate originally' is the wordplay.
'river' becomes 'dee' (River Dee).
'originally' indicates taking the first letters.
The first letter of 'plate' is 'p'.
'dee'+'p'='DEEP'
(Other definitions for deep that I've seen before include "hard to understand" , "Wise - difficult to understand" , "profoundly" , "Main" , "dangerous for swimming?" .)