Grin: note fluffed is a gas (8)
I believe the answer is:
'gas' is the definition.
(I know that nitrogen is a type of gas)
'grin note fluffed' is the wordplay.
'grin note' can be anagrammed to 'NITROGEN'.
However, I'm not clear how the anagram is indicated.
'is a' is the link.
Can you help me to learn more?
(Other definitions for nitrogen that I've seen before include "Gas forming 78% of the air" , "Commonest atmospheric gas" , "It's a gas, the strange ringtone" , "Most prevalent gas in air" , "Gas forming about four fifths of the atmosphere" .)