Seat is bad for sleep (6)
I believe the answer is:
siesta
'sleep' is the definition.
(I've seen this before)
'seat is bad' is the wordplay.
'bad' indicates an anagram.
'seat'+'is'='seatis'
'seatis' with letters rearranged gives 'SIESTA'.
'for' is the link.
(Other definitions for siesta that I've seen before include "Daytime nap" , "Seat is found for a Spanish rest period" , "Catnap" , "Afternoon snooze" , "rest of the Spaniards" .)