Knight's advance across wide area (6)
I believe the answer is:
gawain
'knight's' is the definition.
(thesaurus)
'advance across wide area' is the wordplay.
'advance' becomes 'gain' ('gain' can be a synonym of 'advance'**).
'across' indicates putting letters inside.
'wide' becomes 'w' (abbreviation e.g. in cricket).
'area' becomes 'a' (maths abbreviation).
'w'+'a'='wa'
'gain' enclosing 'wa' is 'GAWAIN'.
(Other definitions for gawain that I've seen before include "K" , "round a table for him and his comrades?" , "Arthurian knight of the Round Table" , "One of King Arthur's knights" , "Man at round table" .)