Pressure peer for gem (5)
I believe the answer is:
pearl
'for gem' is the definition.
(pearl is a kind of gem)
'pressure peer' is the wordplay.
'pressure' becomes 'p' (scientific abbreviation).
'peer' becomes 'earl' (earl is a kind of peer).
'p'+'earl'='PEARL'
(Other definitions for pearl that I've seen before include "See 12" , "prized example" , "Object of great worth" , "Gem from the sea" , "Precious stone from shellfish" .)