Incarcerate one by one, say (6)
I believe the answer is:
intern
'incarcerate' is the definition.
(thesaurus)
'one by one say' is the wordplay.
'one by one' becomes 'in turn' ('in turn' means one by one).
'say' indicates a 'sounds like' (homophone) clue.
'inturn' sounds like 'INTERN'.
(Other definitions for intern that I've seen before include "Young American 18" , "Novice gaining experience" , "Lock up the hospital doctor" , "Confine - physician in US hospital" , "Confine - person on work experience" .)