Lock up one by one, say (6)
I believe the answer is:
intern
'lock up' is the definition.
(thesaurus)
'one by one say' is the wordplay.
'one by one' becomes 'in turn' ('in turn' means one by one).
'say' shows a homophone (sound like).
'inturn' is a homophone of 'INTERN'.
(Other definitions for intern that I've seen before include "Hold in custody" , "Imprison without trial - hospital junior doctor" , "Imprison (enemy aliens)" , "put in jail" , "House officer" .)