Belt tune out — an aid to digestion! (5,3)
I believe the answer is:
betel nut
'aid to digestion' is the definition.
I can't judge whether this definition defines the answer.
'belt tune out' is the wordplay.
'out' is an anagram indicator (out can mean wrong or inaccurate).
'belt'+'tune'='belttune'
'belttune' is an anagram of 'BETEL-NUT'.
'an' is the link.
Can you help me to learn more?
(Other definitions for betel nut that I've seen before include "stimulant" , "that's found on palm" , "supplies a narcotic" , "Seed of a palm chewed (with its leaves) as a narcotic" .)