Part of ulcer is extremely red (6)
I believe the answer is:
cerise
'red' is the definition.
(I've seen this before)
'part of ulcer is extremely' is the wordplay.
'part of' indicates a hidden word.
'CERISE' can be found hidden inside 'ulcer is extremely'.
(Other definitions for cerise that I've seen before include "Bright cherry-red colour" , "Vivid reddish-pink" , "Pinkish red" , "Light purplish red" , "'E cries at the colour red" .)