Soup beginning to inflame large mouth ulcer (7)
I believe the answer is:
gumboil
'ulcer' is the definition.
(thesaurus)
'soup beginning to inflame large mouth' is the wordplay.
'soup' becomes 'gumbo' (type of Cajun soup).
'beginning to' suggests taking the first letters.
'mouth' says to take the initial letters (mouth can mean something's entrance).
The first letter of 'inflame' is 'i'.
The first letter of 'large' is 'l'.
'gumbo'+'i'+'l'='GUMBOIL'
(Other definitions for gumboil that I've seen before include "Swelling in the mouth" , "Small abscess of the mouth" .)