He works with locks (6)
I believe the answer is:
barber
'he works with locks' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'he works with' is the wordplay.
'he' becomes 'Bar'.
'works with' becomes 'ber' (I am not sure about this - if you are sure you should believe this answer much more).
'bar'+'ber'='BARBER'
Can you help me to learn more?
(Other definitions for barber that I've seen before include "Hairdresser for men - musically from Seville" , "Traditional men's hairdresser" , "Rossini's opera 'The . . . . . . of Seville'" , "responsible for Hair?" , "Men's hairdresser, musically of Seville" .)