Large amount, halibut ultimately consumed by whale at sea (6)
I believe the answer is:
wealth
'large amount' is the definition.
'wealth' can be an answer for 'amount' (wealth is a kind of amount). I'm unsure of the 'large' bit.
'halibut ultimately consumed by whale at sea' is the wordplay.
'ultimately' indicates one should take the final letters.
'consumed by' means one lot of letters goes inside another (inserted letters get consumed).
'at sea' indicates anagramming the letters.
The last letter of 'halibut' is 't'.
'whale' anagrammed gives 'wealh'.
't' going into 'wealh' is 'WEALTH'.
Can you help me to learn more?
(Other definitions for wealth that I've seen before include "'Riches, affluence (6)'" , "Riches, worldly goods" , "Prosperity" , "was common under Cromwell" , "Material worth" .)