Joined league's leader during season, top of division (7)
I believe the answer is:
spliced
'joined' is the definition.
(splicing is a kind of joining)
'league's leader during season top of division' is the wordplay.
'leader' says to take the initial letters.
'during' is an insertion indicator.
'season' becomes 'spice' (spicing is a kind of seasoning).
'top of' says to take the initial letters.
The first letter of 'leagues' is 'l'.
The first letter of 'division' is 'd'.
'l' put inside 'spice' is 'splice'.
'splice'+'d'='SPLICED'
(Other definitions for spliced that I've seen before include "Joined two ends of rope together by weaving" , "Like a cricket bat" , "in a match" , "Wed" , "get married" .)