Flab-tightening procedure for corpulent friar (4)
I believe the answer is:
tuck
I believe this is a double definition.
'flab-tightening procedure' is the first definition.
(cosmetic surgery)
'corpulent friar' is the second definition.
(in Robin Hood)
'for' acts as a link.
(Other definitions for tuck that I've seen before include "Flattened pleat stitched in place" , "Fold into small space" , "Cosmetic procedure" , "Schoolboy grub like sweets and cakes" , "Dart" .)