Unknown marvel not initially there (6)
I believe the answer is:
yonder
'there' is the definition.
(I've seen this before)
'unknown marvel not initially' is the wordplay.
'unknown' becomes 'y' (algebra).
'marvel' becomes 'wonder' (synonyms).
'not initially' means to remove the first letter.
'wonder' with its first letter taken off is 'onder'.
'y'+'onder'='YONDER'
(Other definitions for yonder that I've seen before include "Distant but within sight" , "In that direction" , "that one" , "See Rodney over there" , "Rodney (anag.)" .)