Lake in which agricultural implement loses power (5)
I believe the answer is:
lough
'lake' is the definition.
(lough is a kind of lake)
'agricultural implement loses power' is the wordplay.
'agricultural implement' becomes 'plough' (I've seen this before).
'loses' is a deletion indicator.
'power' becomes 'p' (symbol in physics).
'plough' with 'p' removed is 'LOUGH'.
'in which' is the link.
(Other definitions for lough that I've seen before include "Example of liquidity" , "Body of water" , "Irish loch" , "Irish word for lake" , "An Irish lake or arm of the sea" .)