Day tours round Apollo's destination (4)
I believe the answer is:
moon
'apollo's destination' is the definition.
(I've seen this in another clue)
'day tours round' is the wordplay.
'day' becomes 'mon' (abbreviation for Monday).
'tours' is an insertion indicator (tour can mean to go around).
'round' becomes 'o' (o is a circular letter).
'mon' placed around 'o' is 'MOON'.
(Other definitions for moon that I've seen before include "It may be full, half or crescent" , "Orbiting body" , "Space traveller" , "High light" , "Satellite of six Down" .)