Recipe for astringent to be put on back (7)
I believe the answer is:
formula
'recipe' is the definition.
(synonyms)
'to be put on back' is the wordplay.
'to' becomes 'for'.
'be put on' becomes 'alum' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'back' shows that the letters should be reversed in order.
'alum' reversed gives 'mula'.
'for'+'mula'='FORMULA'
'for astringent' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for formula that I've seen before include "Mathematical expression for solving problems" , "Group of symbols making a mathematical statement" , "equation" , "Category in motor racing" , "Baby food" .)