Lock up one by one, we hear (6)
I believe the answer is:
intern
'lock up' is the definition.
(thesaurus)
'one by one we hear' is the wordplay.
'one by one' becomes 'in turn' ('in turn' means one by one).
'we hear' indicates a 'sounds like' (homophone) clue.
'inturn' is a homophone of 'INTERN'.
(Other definitions for intern that I've seen before include "Work experience trainee" , "Confine the hospital resident" , "Novice gaining experience" , "Lock up the hospital doctor" , "Confine - physician in US hospital" .)