Wonder when five enter fantastic realm (6)
I believe the answer is:
marvel
'wonder' is the definition.
(I know that wonder can be written as marvel)
'five enter fantastic realm' is the wordplay.
'five' becomes 'V' (Roman numeral).
'enter' means one lot of letters goes inside another.
'fantastic' is an anagram indicator (a fantastic or improbable ordering of the letters).
'realm' is an anagram of 'marel'.
'v' inserted within 'marel' is 'MARVEL'.
'when' is the link.
(Other definitions for marvel that I've seen before include "Wonderful thing/person" , "Gaze, wonder" , "Wonder at, be amazed" , "Something very wonderful" , "'Wonder, miracle (6)'" .)