Outcome of evacuating 50% of underground jail cells? (4)
I believe the answer is:
dung
'outcome of evacuating' is the definition.
(evacuate can mean excrete)
'50% of underground jail cells?' is the wordplay.
'50% of' means to take half the letters.
'underground jail cells?' becomes 'dungeons'.
'dungeons' cut in half is 'DUNG'.
(Other definitions for dung that I've seen before include "Animal excrement" , "Animal manure" .)