Firm from Germany provides secret writing (4)
I believe the answer is:
code
'secret writing' is the definition.
(I've seen this in another clue)
'firm from germany' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'from' says to put letters next to each other.
'germany' becomes 'de' (I've seen this in other clues).
'co'+'de'='CODE'
'provides' acts as a link.
(Other definitions for code that I've seen before include "Kind of encryption designed to disguise content" , "Ethical standards" , "See 12" , "System used to disguise or represent words, like Morse" , "Program instruction" .)