Couple study a length of part of digestive system (8)
I believe the answer is:
duodenal
'part of digestive system' is the definition.
(relating to the duodenum, part of small intestine)
'couple study a length' is the wordplay.
'couple' becomes 'duo' (synonyms).
'study' becomes 'den' (term for a study or office).
'length' becomes 'l'.
'duo'+'den'+'a'+'l'='DUODENAL'
'of' is the link.
(Other definitions for duodenal that I've seen before include "Relating to part of the small intestine" , "Of part of the small intestine, location of an ulcer perhaps" , "Description of piece of inner tube" .)