Choose to interrupt odd dieter who's a fanatic (7)
I believe the answer is:
devotee
'a fanatic' is the definition.
(I've seen this in another clue)
'choose to interrupt odd dieter' is the wordplay.
'choose' becomes 'vote' (voting is a kind of choosing).
'to interrupt' is an insertion indicator.
'odd' means one should take alternating letters.
The alternate letters of 'dieter' are 'dee'.
'vote' placed within 'dee' is 'DEVOTEE'.
'who's' acts as a link.
(Other definitions for devotee that I've seen before include "An enthusiastic admirer and supporter" , "Keen fan" , "A very loyal or loving person" , "One strongly interested in something" , "Great fan" .)