Groucho finally put on Zeppo's coat with the essence of manic energy - it's a gas (5)
I believe the answer is:
ozone
'a gas' is the definition.
(ozone is a kind of gas)
'groucho finally put on zeppo's coat with the essence of manic energy' is the wordplay.
'finally' says to take the final letters.
'put on' says to put letters next to each other.
'coat' suggests removing the centre (only the letters which coat the word are used).
'with' means one lot of letters go next to another.
'the essence of manic energy' becomes 'ne' (I can't justify this - if you can you should give a lot more credence to this answer).
'zeppo' with its centre removed is 'zo'.
The last letter of 'groucho' is 'o'.
'o'+'zo'+'ne'='OZONE'
'it's' acts as a link.
(Other definitions for ozone that I've seen before include "Less common form of oxygen" , "A poisonous gas; seaside air" , "Sea air; toxic gas" , "Colourless gas soluble in alkalis and cold water" , "It's said to brace" .)