Pad up tam with hair (8)
I believe the answer is:
mattress
'pad' is the definition.
(I've seen this before)
'up tam with hair' is the wordplay.
'up' is a reversal indicator (in down clue: letters go upwards).
'with' says to put letters next to each other.
'hair' becomes 'tress' (tress is a kind of hair**).
'tam' back-to-front is 'mat'.
'mat'+'tress'='MATTRESS'
(Other definitions for mattress that I've seen before include "Palliasse" , "Soft part of bed" , "Stuffed bag on which to sleep" , "Sleeping support" , "For the smart set to lie on?" .)