Fancy lung? A haggis, that'll put a smile on your face (8,3)
I believe the answer is:
laughing gas
'that'll put a smile on your face' is the definition.
The definition suggests a singular noun which matches the answer.
'fancy lung? a haggis' is the wordplay.
'fancy' indicates an anagram.
'lung'+'a'+'haggis'='lungahaggis'
'lungahaggis' is an anagram of 'LAUGHING-GAS'.
Can you help me to learn more?
(Other definitions for laughing gas that I've seen before include "Name for nitrous oxide" , "Old number" , "Early anaesthetic; nitrous oxide" , "it'll knock you out" , "Mild anaesthetic used in dentistry and surgery" .)