I hold back, assuming it’s begun (9)
I believe the answer is:
initiated
'begun' is the definition.
(I've seen this before)
'i hold back assuming it's' is the wordplay.
'hold' becomes 'detain' (I've seen this in another clue).
'back' says the letters should be written backwards.
'assuming' indicates putting letters inside (inserted letters are taken in or assumed).
'detain' in reverse letter order is 'niated'.
'niated' going around 'it' is 'nitiated'.
'i'+'nitiated'='INITIATED'
(Other definitions for initiated that I've seen before include "Begun" , "Those admitted into the mysteries" , "Accepted into a group by way of a ritual" , "Started" , "Began" .)