Hate losing sulphate? On reflection, it's a good thing! (4)
I believe the answer is:
plus
'a good thing' is the definition.
(I've seen this in another clue)
'hate losing sulphate? on reflection' is the wordplay.
'losing' is a deletion indicator.
'on reflection' shows that the letters should be reversed in order.
'sulphate' with 'hate' removed is 'sulp'.
'sulp' written backwards gives 'PLUS'.
'it's' acts as a link.
(Other definitions for plus that I've seen before include "being a little cross" , "mark for arithmetic" , "The opposite of minus" , "To the French it's more" , "Furthermore" .)