Layer of coal in same form (4)
I believe the answer is:
seam
'layer of coal' is the definition.
(I've seen this in another clue)
'same form' is the wordplay.
'form' is an anagram indicator (another form of the letters).
'same' anagrammed gives 'SEAM'.
'in' acts as a link.
(Other definitions for seam that I've seen before include "Stratum (of a mineral)" , "Join - stratum of ore" , "Stratum of ore or coal" , "Join round cricket ball" , "Fabric joint" .)