As potential purchaser, gets into stonier ground (5,2)
I believe the answer is:
tries on
'as potential purchaser gets' is the definition.
I can't judge whether this definition defines the answer.
'stonier ground' is the wordplay.
'ground' is an anagram indicator ('grind' the letters into a new form).
'stonier' anagrammed gives 'TRIES ON'.
'into' is the link.
Can you help me to learn more?
(Other definitions for tries on that I've seen before include "Dons clothes in shop" , "tests" .)