Fish left in sea, somewhat lacking? (6)
I believe the answer is:
marlin
'fish left' is the definition.
'marlin' can be an answer for 'fish' (I've seen this before). I am unsure of the 'left' bit.
'in sea somewhat lacking?' is the wordplay.
'in' is an insertion indicator.
'sea' becomes 'main' ('main' can be a synonym of 'sea').
'somewhat lacking?' becomes 'rl' (I can't justify this - if you can you should give a lot more credence to this answer).
'main' placed around 'rl' is 'MARLIN'.
Can you help me to learn more?
(Other definitions for marlin that I've seen before include "Prized game fish" , "Large game fish" , "Sea roamer" , "type of fish" , "Large ocean fish" .)