Choose sulphur to put in (6)
I believe the answer is:
select
'choose' is the definition.
(I know that choose can be written as select)
'sulphur to put in' is the wordplay.
'sulphur' becomes 'S' (synonyms).
'to put in' becomes 'elect' (I've seen this in another clue).
's'+'elect'='SELECT'
(Other definitions for select that I've seen before include "'Choose, pick out (6)'" , "Choice, picked out" , "Single out" , "Choose, decide" , "Carefully chosen; choose" .)