Letter and next two found between opposing letters in manuscript (5)
I believe the answer is:
codex
'manuscript' is the definition.
(codex is a kind of manuscript)
'letter and next two found between opposing letters' is the wordplay.
'letter' becomes 'x'.
'and' means one lot of letters go next to another.
'next two found between opposing letters' becomes 'code' (I can't explain this - if you can you should believe this answer much more).
'x' after 'code' is 'CODEX'.
'in' is the link.
Can you help me to learn more?
(Other definitions for codex that I've seen before include "Volume of ancient manuscripts" , "Book of ancient manuscripts" .)