Miserable person, head down to go to be sick (5)
I believe the answer is:
retch
'to go to be sick' is the definition.
(to retch is to dry heave)
'miserable person head down' is the wordplay.
'miserable person' becomes 'wretch' (term for a unfortunate person).
'head down' means to remove the first letter (the head of the word is removed).
'wretch' with its first letter taken off is 'RETCH'.
(Other definitions for retch that I've seen before include "Strain as if to vomit" , "Try to bring up" , "Strain to be sick" , "Unsuccessful effort to vomit" , "throw up" .)