Recipe adapted to double sulphate in retrospect (7)
I believe the answer is:
formula
'recipe' is the definition.
('formula' can be a synonym of 'recipe')
'to double sulphate in retrospect' is the wordplay.
'to' becomes 'for'.
'double sulphate' becomes 'alum' (I've seen this before).
'in retrospect' is a reversal indicator.
'alum' in reverse letter order is 'mula'.
'for'+'mula'='FORMULA'
'adapted' acts as a link.
This may not be right. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for formula that I've seen before include "One that's on track" , "Recipe, prescribed form" , "Symbols defining chemical composition" , "Group of symbols expressing a rule" , "Set form of words or mehod" .)