Excessive exploitation very nearly blocks river (7)
I believe the answer is:
overuse
'excessive exploitation' is the definition.
(using or exploiting excessively)
'very nearly blocks river' is the wordplay.
'nearly' means to remove the last letter.
'blocks' means one lot of letters goes inside another (upper walls off glass inside formal).
'river' becomes 'ouse' (Yorkshire river).
'very' with its final letter removed is 'ver'.
'ver' inserted within 'ouse' is 'OVERUSE'.
(Other definitions for overuse that I've seen before include "Excessive (drug) taking" , "Excessively utilise" , "Employ excessively" , "Strain" , "Too much work" .)