To take effect, archbishop's focus must be on essence of volunteering (4)
I believe the answer is:
bite
'to take effect' is the definition.
(I've seen this before)
'archbishop's focus must be on essence of volunteering' is the wordplay.
'focus' says to take the centre (focus can mean the centre of something).
'must be on' is a charade indicator (letters next to each other).
'essence of' says to take the centre.
The middle of 'archbishop' is 'bi'.
The middle letters of 'volunteering' are 'te'.
'bi'+'te'='BITE'
(Other definitions for bite that I've seen before include "The taking of bait by a fish" , "Snake or insect wound" , "Light meal" , "Quick snack" , "Mouthful" .)