Supporters, lots getting hard facts (5,5)
I believe the answer is:
'hard facts' is the definition.
(I've seen this before)
'supporters lots getting' is the wordplay.
'supporters' becomes 'bras' (I've seen this in other clues).
'lots' becomes 'stacks' ('stack' can be a synonym of 'lot'**).
'getting' says to put letters next to each other.
(Other definitions for brass tacks that I've seen before include "Basic facts (colloq.)" , "Hard facts" , "Basic realities (informal)" , "Real business" , "really mean business" .)