Pressure to peer for a gem (5)
I believe the answer is:
pearl
'for a gem' is the definition.
(pearl is a kind of gem)
'pressure to peer' is the wordplay.
'pressure' becomes 'p' (scientific abbreviation).
'to peer' becomes 'earl' (earl is a kind of peer).
'p'+'earl'='PEARL'
(Other definitions for pearl that I've seen before include "Off-white colour" , "Lustrous structure in a mollusc shell" , "Gem from the sea" , "finest example" , "something highly valued" .)