Father covering round behind (3)
I believe the answer is:
for
'behind' is the definition.
(I've seen this in another clue)
'father covering round' is the wordplay.
'father' becomes 'fr' (abbreviation as in a priest's title).
'covering' means one lot of letters goes inside another.
'round' becomes 'o' (round can mean a round object).
'fr' enclosing 'o' is 'FOR'.
(Other definitions for for that I've seen before include "In favour of; since" , "See 24" , "and 16 and 20: Notwithstanding" , "Favouring" , "Pro" .)