Underwear piles - the basic facts (5,5)
I believe the answer is:
brass tacks
'the basic facts' is the definition.
(I've seen this before)
'underwear piles' is the wordplay.
'underwear' becomes 'bras'.
'piles' becomes 'stacks' (stack is a kind of pile).
'bras'+'stacks'='BRASS TACKS'
(Other definitions for brass tacks that I've seen before include "Basics" , "really mean business" , "Business one gets down to" , "Basic realities (informal)" , "Hard facts" .)