Vessel in a lake, generally (3)
I believe the answer is:
keg
'vessel' is the definition.
(I know that keg is a type of vessel)
'in a lake generally' is the wordplay.
'in' says the answer is hidden in the clue.
'KEG' can be found hidden inside 'lake generally'.
(Other definitions for keg that I've seen before include "Small cask or barrel" , "Small beer cask" , "What's found at King's Head" , "Sort of beer" , "Barrel as for beer" .)