Intern beginning to cotton on OK (7)
I believe the answer is:
confine
'intern' is the definition.
(thesaurus)
'beginning to cotton on ok' is the wordplay.
'beginning to' indicates taking the first letters.
'ok' becomes 'fine' ('fine' can be a synonym of 'ok').
The initial letter of 'cotton' is 'c'.
'c'+'on'+'fine'='CONFINE'
(Other definitions for confine that I've seen before include "Keep within limits" , "Imprison" , "Incarcerate" , "Restrict (to an area)" , "Limit" .)