Happening to commit foul at snooker, pocketing green initially (5,3)

'happening' is the definition.
(go off can mean to happen or take place)

'commit foul at snooker pocketing green initially' is the wordplay.
'commit foul at snooker' becomes 'go in off' (going in off is a type of foul in snooker).
'pocketing' is an insertion indicator.
'initially' says to take the initial letters.
The first letter of 'green' is 'g'.
'goinoff' placed around 'g' is 'GOING OFF'.

'to' is the link.