Surplus sulphur inside is scanty (6)
I believe the answer is:
sparse
'is scanty' is the definition.
(I've seen this in another clue)
'surplus sulphur inside' is the wordplay.
'surplus' becomes 'spare' (synonyms).
'sulphur' becomes 'S' (synonyms).
'inside' indicates putting letters inside.
'spare' enclosing 's' is 'SPARSE'.
(Other definitions for sparse that I've seen before include "Thinly-spread" , "Infrequent" , "Thinly dotted about" , "Not much" , "Meagre; scarce" .)