Gem puts pressure on aristocrat (5)
I believe the answer is:
pearl
'gem' is the definition.
(I know that pearl is a type of gem)
'pressure on aristocrat' is the wordplay.
'pressure' becomes 'p' (abbreviation).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'aristocrat' becomes 'Earl' (earl is a kind of aristocrat).
'p'+'earl'='PEARL'
'puts' is the link.
(Other definitions for pearl that I've seen before include "coming out of shell?" , "Drop" , "Paragon" , "Smooth lustrous round structure of calcium carbonate" , "Precious stone from shellfish" .)