Test paper with its own solution (6)
I believe the answer is:
litmus
'test paper' is the definition.
(litmus is a kind of test paper)
'its own solution' is the wordplay.
I cannot quite understand how this works, but
'its' could be 'it' (I've seen this in other clues) and 'it' is located in the answer.
This explanation may well be incorrect...
'with' acts as a link.
Can you help me to learn more?
(Other definitions for litmus that I've seen before include "Substance used to distinguish between acid and alkali" , "Material from lichens used to test for acidity" , "Substance used to tell acid from alkali" , "Colouring material used as a rough acid-base indicator" , "Substance used in test for acidity" .)