Relatively rare argon put in high pressures certainly having leaked (7)
I believe the answer is:
sparser
'relatively rare' is the definition.
(sparse can mean rare)
'argon put in high pressures certainly having leaked' is the wordplay.
I cannot really see how this works, but
'argon' could be 'ar' (Ar is the chemical symbol for argon) and 'ar' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for sparser that I've seen before include "Less thick" , "not so common" .)