Club supporter's leader that is supporting supporters (7)
'club' is the definition.
(brassie is a kind of club)
'supporter's leader that is supporting supporters' is the wordplay.
'leader' suggests taking the first letters.
'that is' becomes 'ie' ('i.e.' can be a synonym of 'that is').
'supporting' means one lot of letters go next to another (in a down clue, some letters go below others).
'supporters' becomes 'bras' (I've seen this in other clues).
The initial letter of 'supporters' is 's'.
'sie' after 'bras' is 'BRASSIE'.
(Other definitions for brassie that I've seen before include "Old golf club", "Golf-club", "Number two wood (golf)", "old wood".)