Fish eating seaweed that’s hypothetically perfect? (5,3)
I believe the answer is:
ideal gas
'hypothetically perfect?' is the definition.
I can't judge whether this defines the answer.
'fish eating seaweed' is the wordplay.
'fish' becomes 'ides' (ide is a type of fish).
'eating' means one lot of letters goes inside another.
'seaweed' becomes 'alga' (singular form of 'algae').
'ides' going around 'alga' is 'IDEAL GAS'.
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for ideal gas that I've seen before include "that's chemically perfect?" , "hypothetical chemical" , "Fluid obeying a theory in chemistry" , "Hypothetical fluid" , "Perfectly-behaving substance in chemistry" .)