Nobleman under pressure seeks gem (5)
I believe the answer is:
pearl
'gem' is the definition.
(I know that pearl is a type of gem)
'nobleman under pressure' is the wordplay.
'nobleman' becomes 'earl' (earl is a kind of nobleman).
'under' means one lot of letters go next to another.
'pressure' becomes 'p' (abbreviation).
'earl' after 'p' is 'PEARL'.
'seeks' is the link.
This may not be right. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for pearl that I've seen before include "Oyster's gem" , "Smooth lustrous round structure of calcium carbonate" , "coming out of shell?" , "greatly valued" , "Lustrous structure in a mollusc shell" .)