Large gnat in mouth could be serious (2,8,6)
I believe the answer is:
no laughing matter
'serious' is the definition.
(thesaurus)
'large gnat in mouth could be' is the wordplay.
'could be' is an anagram indicator.
'large'+'gnat'+'in'+'mouth'='largegnatinmouth'
'largegnatinmouth' with letters rearranged gives 'NO LAUGHING MATTER'.
(Other definitions for no laughing matter that I've seen before include "A very serious affair" , "A very serious thing" .)