Worst possible sources of coal (4)
I believe the answer is:
pits
'worst' is the definition.
(I've seen this in another clue)
'possible sources of coal' is the wordplay.
'sources of' says to take the initial letters.
'coal' becomes 'its' (I am not sure about this - if you are sure you should believe this answer much more).
The initial letter of 'possible' is 'p'.
'p'+'its'='PITS'
Can you help me to learn more?
(Other definitions for pits that I've seen before include "Mines" , "Deep holes in the ground" .)