Business details underwear mountains (5,5)
I believe the answer is:
brass tacks
'business details' is the definition.
'underwear mountains' is the wordplay.
'underwear' becomes 'bras'.
'mountains' becomes 'stacks'.
'bras'+'stacks'='BRASS TACKS'
(Other definitions for brass tacks that I've seen before include "Basic facts (colloq.)" , "Basic realities (informal)" , "Business one gets down to" , "really mean business" , "Real business" .)