Bad day switching round sink (5)
I believe the answer is:
droop
'sink' is the definition.
(to droop is to sink or sag)
'bad day switching round' is the wordplay.
'bad' becomes 'poor' (similar in meaning).
'day' becomes 'd' (abbreviation for day).
'switching round' shows that the letters should be reversed in order.
'poor'+'d'='poord'
'poord' reversed gives 'DROOP'.
(Other definitions for droop that I've seen before include "languish" , "be despondent" , "Become limp" , "Sag, wilt" , "Decline" .)