Black stuff in bottomless lake (3)
I believe the answer is:
tar
'black stuff' is the definition.
(tar is a black substance)
'bottomless lake' is the wordplay.
'bottomless' means to remove the last letter (the letter at the bottom is removed).
'lake' becomes 'tarn' (tarn is a kind of lake).
'tarn' with its final letter taken off is 'TAR'.
'in' is the link.
(Other definitions for tar that I've seen before include "It paves the way" , "Old salt, sailor" , "Road-dressing for a sailor?" , "Viscous mixture - sailor" , "Informal name for sailor" .)