Even witnesses testify initially to a map in a map (5)
I believe the answer is:
inset
'a map in a map' is the definition.
(I've seen this in another clue)
'even witnesses testify initially' is the wordplay.
'even' means one should take alternating letters (even-numbered letters are used).
'initially' suggests taking the first letters.
The initial letter of 'testify' is 't'.
The alternate letters of 'witnesses' are 'inse'.
'inse'+'t'='INSET'
'to' acts as a link.
(Other definitions for inset that I've seen before include "Small map within another" , "Piece sewn into a garment" , "Part of magazine" , "Extra page or piece added to enlarge" , "Picture placed within another" .)