Some acid gets registered in caper (6)
I believe the answer is:
frolic
'caper' is the definition.
(synonyms)
'some acid gets registered' is the wordplay.
I cannot quite see how this works, but
'registered' could be 'r' (abbreviation, as in registered trademark) and 'r' is located in the answer.
The remaining letters 'folic' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'in' is the link.
Can you help me to learn more?
(Other definitions for frolic that I've seen before include "Caper, gambol" , "Move in a cheerful and lively way" , "Gambol; prank" , "Merrymaking" , "fun" .)