Recalled bed - nasty thing, the alarm (6)
I believe the answer is:
tocsin
'the alarm' is the definition.
(tocsin is a kind of alarm)
'recalled bed nasty thing' is the wordplay.
'recalled' shows that the letters should be reversed in order.
'bed' becomes 'cot' (cot is a kind of bed).
'nasty thing' becomes 'sin'.
'cot' reversed gives 'toc'.
'toc'+'sin'='TOCSIN'
(Other definitions for tocsin that I've seen before include "Warning bell" , "One sounds an alarm" , "The sound of an alarm bell" , "An alarm bell or signal" , "alarm call" .)