Pressure nobleman for a jewel (5)
I believe the answer is:
pearl
'a jewel' is the definition.
(pearl is a kind of jewel)
'pressure nobleman' is the wordplay.
'pressure' becomes 'p' (abbreviation).
'nobleman' becomes 'earl' (earl is a kind of nobleman).
'p'+'earl'='PEARL'
'for' is the link.
(Other definitions for pearl that I've seen before include "finest example" , "Precious gem" , "greatly valued" , "Small type" , "Girl < , pretty" .)