Mainly leading role around Thursday (3,3,4,4)
I believe the answer is:
for the most part
'mainly' is the definition.
(thesaurus)
'leading role around thursday' is the wordplay.
'leading' becomes 'foremost' (I've seen this before**).
'role' becomes 'part' ('part' can be a synonym of 'role').
'around' indicates putting letters inside.
'thursday' becomes 'th' ('Th' can be a synonym of 'thursday').
'foremost'+'part'='foremostpart'
'foremostpart' placed around 'th' is 'FOR THE MOST PART'.
(Other definitions for for the most part that I've seen before include "Usually" , "Chiefly" , "in general" , "Mainly" .)