Johnson's written about Old English verse (4)
I believe the answer is:
poem
'verse' is the definition.
(I've seen this in another clue)
'johnson's written about old english' is the wordplay.
'johnson' becomes 'pm' (prime minister Boris Johnson).
'written about' indicates putting letters inside.
'old english' becomes 'oe'.
'pm' enclosing 'oe' is 'POEM'.
(Other definitions for poem that I've seen before include "Verse form" , "Mope (anag.)" , "Literary composition, sometimes in verse" , "Piece of verse" , "Mope about in verse" .)