Old language, best used by God (catholic) (7)
I believe the answer is:
aramaic
'old language' is the definition.
'aramaic' can be an answer for 'language' (Aramaic is an example). I am not sure about the 'old' bit.
'best used by god catholic' is the wordplay.
'best' becomes 'a'.
'used' becomes 'mai' (I can't explain this - if you can you should believe this answer much more).
'by' means one lot of letters go next to another.
'god' becomes 'Ra' (Egyptian god).
'catholic' becomes 'C' (abbreviation as in RC for Roman Catholic).
'mai' put after 'ra' is 'ramai'.
'a'+'ramai'+'c'='ARAMAIC'
Can you help me to learn more?
(Other definitions for aramaic that I've seen before include "Semitic language spoken in New Testament times" , "Near Eastern language" , "Ancient Semitic language" , "Old Middle-Eastern language" , "Biblical language" .)