Constant study arranged around start of ordinary detention (7)
I believe the answer is:
custody
'detention' is the definition.
(I know that detention can be written as custody)
'constant study arranged around start of ordinary' is the wordplay.
'constant' becomes 'c' (common symbol in maths for an unknown constant).
'arranged' indicates anagramming the letters (letters rearranged).
'around' indicates putting letters inside.
'start of' indicates taking the first letters.
The first letter of 'ordinary' is 'o'.
'study' anagrammed gives 'ustdy'.
'ustdy' placed around 'o' is 'ustody'.
'c'+'ustody'='CUSTODY'
(Other definitions for custody that I've seen before include "lock-up?" , "Protective care" , "Keeping - detention" , "State of being detained by the police" , "One of spelt out words" .)