Red gem in a seawater fish trap (6)
I believe the answer is:
garnet
'red gem' is the definition.
(I've seen this in another clue)
'fish trap' is the wordplay.
'fish' becomes 'gar' (type of gar).
'trap' becomes 'net' (net is a type of trap).
'gar'+'net'='GARNET'
'in a seawater' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for garnet that I've seen before include "January birthstone" , "Semi-precious stone of argent type" , "Deep red precious stone" , "Semi-precious stone of deep transparent red" , "Deep red gem stone" .)