In fact, an essay about an upper-class Labour leader (5)
I believe the answer is:
truly
'in fact' is the definition.
(I've seen this in another clue)
'an essay about an upper-class labour leader' is the wordplay.
'an essay' becomes 'try' (I've seen this in another clue).
'about' is an insertion indicator.
'an upper class' becomes 'u' (as in U- and non-U speech).
'labour leader' becomes 'l' (the leading letter of 'labour').
'u'+'l'='ul'
'try' enclosing 'ul' is 'TRULY'.
(Other definitions for truly that I've seen before include "Really, genuinely" , "with accuracy" , "Indeed" , "Genuinely, properly" , "Verily" .)