Truth of the matter, knickers finally found in piles of underwear? (5,5)
I believe the answer is:
brass tacks
'truth of the matter' is the definition.
(as in 'let's get down to brass tacks')
'knickers finally found in piles of underwear?' is the wordplay.
'finally' suggests the final letters.
'found in' indicates putting letters inside.
'piles of underwear?' becomes 'brastacks' (stacks of bras).
The final letter of 'knickers' is 's'.
's' going inside 'brastacks' is 'BRASS TACKS'.
(Other definitions for brass tacks that I've seen before include "Basic realities (informal)" , "Details of practical business (colloq.)" , "Real business" , "really mean business" , "Basics" .)