Part of digestive system in trial Dionne regularly observed (9)
I believe the answer is:
intestine
'part of digestive system' is the definition.
('intestine' is part of 'digestive system')
'in trial dionne regularly observed' is the wordplay.
'trial' becomes 'test' (synonyms).
'regularly observed' indicates alternate letters (letters selected at regular intervals).
The alternating letters of 'dionne' are 'ine'.
'in'+'test'+'ine'='INTESTINE'
(Other definitions for intestine that I've seen before include "Part of the alimentary canal" , "Being gutted, one loses it" , "canal section" , "Some guts" .)