Supplement part of the digestive system (8)
I believe the answer is:
appendix
'supplement part of the digestive system' is the definition.
'appendix' can be an answer for 'supplement' (I've seen this in another clue). I am unsure of the 'part of the digestive system' bit.
'supplement part of the digestive system' is the wordplay.
'supplement' becomes 'append' (synonyms).
'part of the digestive system' becomes 'ix' (I can't justify this - if you can you should believe this answer much more).
'append'+'ix'='APPENDIX'
Can you help me to learn more?
(Other definitions for appendix that I've seen before include "part of book" , "sometimes removed?" , "often discarded in theatre" , "It contains supplementary material" , "Writer's addition - may be taken out in the theatre!" .)