It's added in front of a word (6)
I believe the answer is:
prefix
'it's added in front of a word' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for prefix that I've seen before include "Element added to the beginning of a word" , "Beginning part of a word" , "put at the front" , "What goes on before" , "Something like ex- or non-" .)