In this puzzle, the second lump! (5)
I believe the answer is:
sugar
This might be a double definition.
'in' is the first definition.
The answer and definition can be both substances as well as being singular nouns.
Perhaps there's an association between them I don't understand?
'puzzle the second lump' is the second definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'this' is the link.
Can you help me to learn more?
(Other definitions for sugar that I've seen before include "Argus (anag.)" , "Sweetening agent" , "Sweet white substance" , "Common sweetener" , "Sweeten" .)