Do fish, later on in the day (8)
I believe the answer is:
eventide
'the day' is the definition.
(eventide is a kind of day)
'do fish later on' is the wordplay.
'do' becomes 'event' (I've seen this in another clue).
'fish' becomes 'ide' (type of fish).
'later on' means one lot of letters go next to another (some letters appear later than others).
'event'+'ide'='EVENTIDE'
'in' is the link.
(Other definitions for eventide that I've seen before include "late in the day?" , "End of the day (poetic)" , "some time later?" , "onset of dusk" , "latter part of the day" .)