Work with printing press: sort firm required (7)
I believe the answer is:
typeset
'work with printing press' is the definition.
(typesetting is arranging text for printing)
'sort firm required' is the wordplay.
'sort' becomes 'type' (type is a kind of sort).
'firm' becomes 'set' (both can mean fixed).
'required' means one lot of letters go next to another.
'type'+'set'='TYPESET'
(Other definitions for typeset that I've seen before include "Arrange (text) for printing" , "(Of fonts) ready to print" , "Like" , "compositor" , "Ready to go public" .)