You must be high if you're breathing this gas! (5)
I believe the answer is:
ozone
'this gas' is the definition.
(I've seen this before)
'you must be high if you're breathing' is the wordplay.
I cannot really see how this works, but
'you' could be 'one' (I've seen this before) and 'one' is present in the answer.
The remaining letters 'oz' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for ozone that I've seen before include "Sea air; bleaching agent" , "By sea, you could take in this" , "Bracing air (colloq.)" , "Gas forming screen against ultraviolet radiation in the stratosphere" , "Allotrope of oxygen" .)