Approximately half circling area (5)
I believe the answer is:
circa
'approximately' is the definition.
(I've seen this in another clue)
'half circling area' is the wordplay.
'half' means to take half the letters.
'area' becomes 'a' (maths abbreviation).
'circling' cut in half is 'circ'.
'circ'+'a'='CIRCA'
(Other definitions for circa that I've seen before include "Approximately, around" , "(Especially of a date) approximately" , "More or less" , "Preceding a date, this means about" , "Preposition meaning about, usually preceding a date" .)