This acid is a vitamin wrapped in foil with a bit of care (5)
I believe the answer is:
folic
'this acid is a vitamin wrapped' is the definition.
The definition suggests an adjective which matches the answer.
'foil with a bit of care' is the wordplay.
I cannot really see how this works, but
an anagram of 'foil' is 'foli' which is present in the answer.
A single letter 'c' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
'in' acts as a link.
Can you help me to learn more?
(Another definition for folic that I've seen is " Deficiency of this acid, of vitamin B group, causes anaemia".)