50 per cent (4)
'50 per cent' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for half that I've seen before include "50 per cent" , "shared round" , "alcoholic drink" , "Short distance - steal!" , "0.5" .)
(Other definitions for semi that I've seen before include "Type of house (UK) - type of truck (US)" , "One of two houses with a common wall" , "Not quite the biggest tie" , "The last but one round" , "Co-joined house" .)