Happen to be starting on a new diet (6)
I believe the answer is:
betide
'happen to' is the definition.
(synonyms)
'be starting on a new diet' is the wordplay.
I cannot really see how this works, but
'be' is found in the answer.
an anagram of 'diet' is 'tide' which is present in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for betide that I've seen before include "Woe - -" , "Happen to (arch)" , "Befall" .)