Venerable historian puts egghead on mattress (4)
I believe the answer is:
bede
'venerable historian' is the definition.
(I've seen this in another clue)
'egghead on mattress' is the wordplay.
'egghead' becomes 'e' (head letter of 'egg').
'on' means one lot of letters go next to another.
'mattress' becomes 'bed' ().
'e' after 'bed' is 'BEDE'.
'puts' is the link.
(Other definitions for bede that I've seen before include "Historian saint; Eliot's Adam" , "Ancient English historian" , "'Venerable' English monk" , "venerable person?" , "Venerable author of The Ecclesiastical History of the English People, d. 735" .)