The compiling of dictionaries (12)
I believe the answer is:
lexicography
'the compiling of dictionaries' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for lexicography that I've seen before include "Process of dictionary compilation" , "The compilation of dictionaries" , "Dictionary compiling" , "Dictionary creation" , "The compiling of dictionaries" .)