Venerable Bede’s lowest is above fair (4)
I believe the answer is:
even
'fair' is the definition.
(thesaurus)
'venerable bede's lowest is above' is the wordplay.
'venerable' becomes 'ven'.
'lowest' says to take the final letters (in a down clue, the bottom-most letter).
'is above' means one lot of letters go next to another (in a down clue, some letters go above others).
The final letter of 'bede' is 'e'.
'ven' after 'e' is 'EVEN'.
(Other definitions for even that I've seen before include "Short joke" , "Like half the numbers" , "Deep crisp snow seen on Boxing Day also this" , "Peaceful" , "Regular; calm" .)