Going off and very quietly supplanting good person in assignment (7)

'going off' is the definition.
(pop can mean to explode or go off)

'very quietly supplanting good person in assignment' is the wordplay.
I cannot really understand how this works, but
'quietly' could be 'pp' and 'pp' is present in the answer.
'good' could be 'g' (abbreviation) and 'g' is found within the answer.
'in' is found in the answer.
The remaining letters 'po' is a valid word which might be clued in a way I don't see.
This may be the basis of clue (or it may be nonsense).

'and' is the link.