Split around one side (5)
I believe the answer is:
cleft
'split' is the definition.
(cleft can mean split or cloven)
'around one side' is the wordplay.
'around' becomes 'c' (abbreviation for 'circa').
'one side' becomes 'left' (as opposed to right).
'c'+'left'='CLEFT'
(Other definitions for cleft that I've seen before include "Narrow gap" , "Fissure in rock" , "Crack" , "Split into two" , "division" .)