Partner cut long hair -- one lying down in the bedroom? (8)
I believe the answer is:
mattress
'one lying down in the bedroom?' is the definition.
(a mattress is laid on a bed)
'partner cut long hair' is the wordplay.
'partner' becomes 'mate' (synonyms).
'cut' means to remove the last letter.
'long hair' becomes 'tress' (tress can mean a lock of hair).
'mate' with its last letter removed is 'mat'.
'mat'+'tress'='MATTRESS'
(Other definitions for mattress that I've seen before include "Large thick pad on the bed" , "Smartest kind of bed-base" , "Pad to sleep on" , "Palliasse" , "Sleeping support" .)