Almost a lump of chemical (4)
I believe the answer is:
alum
'chemical' is the definition.
The answer and definition can be both substances as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'almost a lump' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'a'+'lump'='alump'
'alump' with its final letter taken away is 'ALUM'.
'of' is the link.
Can you help me to learn more?
(Other definitions for alum that I've seen before include "Compound used in tanning and dying" , "Colourless compound used in dyeing and tanning" , "Compound used in dyeing and to stop bleeding" , "Sulphate used in dyeing and tanning" , "Potassium compound; IoW sandy bay" .)