Half (4)
(Other definitions for half that I've seen before include "Moiety" , "Short distance - steal!" , "Two of these to the pint" , "proportion" , "one over a couple" .)
'half' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for semi that I've seen before include "Not detached house" , "Common type of house" , "House attached to one other" , "House joined to another" , "Partly detached home" .)