Constant area for one to circulate in prison (4)
I believe the answer is:
cage
'prison' is the definition.
(cage is a kind of prison)
'constant area for one to circulate' is the wordplay.
'constant' becomes 'c' (mathematical abbreviation).
'area' becomes 'a' (maths abbreviation).
'for one' becomes 'eg' (both can mean 'for example').
'to circulate' shows that the letters should be reversed in order (I've seen 'circulating' mean this).
'eg' back-to-front is 'ge'.
'c'+'a'+'ge'='CAGE'
'in' is the link.
(Other definitions for cage that I've seen before include "American composer with centenary this year" , "Prison for 21" , "Shut up!" , "Confine -- US composer, d. 1992" , "Confine behind bars" .)