What some seams are made of (4)
I believe the answer is:
coal
'made of' is the definition.
The definition suggests a singular noun which matches the answer.
'what some seams' is the wordplay.
I cannot quite understand how this works, but
'some' could be 'a' and 'a' is present in the answer.
The remaining letters 'col' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'are' is the link.
Can you help me to learn more?
(Other definitions for coal that I've seen before include "Carbon fuel" , "Mined fuel" , "Fossil fuel" , "Char" , "Black mineral used for burning" .)