Flower yield reduced when given copper and sulphur (6)
I believe the answer is:
crocus
'flower' is the definition.
(I've seen this before)
'reduced when given copper and sulphur' is the wordplay.
'reduced' becomes 'cro' (I can't justify this - if you can you should believe this answer much more).
'when given' means one lot of letters go next to another.
'copper' becomes 'Cu' (Cu is the chemical symbol for copper).
'and' is a charade indicator (letters next to each other).
'sulphur' becomes 'S' (synonyms).
'cro'+'cu'+'s'='CROCUS'
'yield' acts as a link.
(Other definitions for crocus that I've seen before include "Small plant with yellow, purple or white flowers" , "Slow transition" , "Spring plant" , "'Spring flower, source of saffron (6)'" , "Plant from a corm" .)