Still activity at sea when the nights are drawing in (9)
I believe the answer is:
eventides
'when the nights are drawing in' is the definition.
(eventide can mean the evening)
'still activity at sea' is the wordplay.
'still' becomes 'even' (synonyms).
'activity at sea' becomes 'tides'.
'even'+'tides'='EVENTIDES'
(Other definitions for eventides that I've seen before include "Heralds of night" , "Late hours" .)