Pipe is back on after measure of acid (6)
I believe the answer is:
siphon
'pipe' is the definition.
(thesaurus)
'back on after measure of acid' is the wordplay.
I cannot quite see how this works, but
'on' is present in the answer.
'measure' could be 'ph' (pH is a kind of measure) and 'ph' is present in the answer.
The remaining letters 'si' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'is' acts as a link.
Can you help me to learn more?
(Other definitions for siphon that I've seen before include "Bottle from which a liquid is forced by pressure of gas" , "Drawing device" , "Soda dispenser" , "Kind of bottle for squirting, say soda" , "Tube used to draw a liquid" .)