Blessing bishop with nothing on (4)
I believe the answer is:
boon
'blessing' is the definition.
('boon' can be a synonym of 'blessing')
'bishop with nothing on' is the wordplay.
'bishop' becomes 'b' (abbreviation used in chess).
'with nothing' becomes 'o' (I've seen this in other clues).
'b'+'o'+'on'='BOON'
(Other definitions for boon that I've seen before include "Beneficial thing" , "Blessing - sort of companion?" , "Benefaction" , "A benefit, goes with blessing" , "Favour, request" .)