Hit when good health reduced by half (4)
I believe the answer is:
chin
'hit' is the definition.
(informal term meaning to punch someone's chin)
'good health reduced by half' is the wordplay.
'good health' becomes 'chinchin' (old-fashioned toast said before drinking).
'reduced by half' means to take half the letters.
'chinchin' halved is 'CHIN'.
'when' acts as a link.
(Other definitions for chin that I've seen before include "Outstanding jaw part" , "Point of the jaw" , "Lower face part" , "Could be a double feature" , "Keep it up and be optimistic" .)