Most of gas in an alcohol is from aquatic plants (5)
I believe the answer is:
algae
'from aquatic plants' is the definition.
I can't judge whether this definition defines the answer.
'most of gas in an alcohol' is the wordplay.
'most of' means to remove the last letter.
'in' means one lot of letters goes inside another.
'an alcohol' becomes 'ale' (ale is a kind of alcohol).
'gas' with its final letter taken away is 'ga'.
'ga' going within 'ale' is 'ALGAE'.
'is' is the link.
Can you help me to learn more?
(Other definitions for algae that I've seen before include "'Simple plants, such as seaweed (5)'" , "Stemless water plants found in a gale" , "Seaweeds generally" , "Primitive organisms" , "Water plants with no true stems or leaves" .)