They drift for the present on board ship (5)
I believe the answer is:
snows
'they drift' is the definition.
(as in snowdrifts)
'the present on board ship' is the wordplay.
'the present' becomes 'now' (now is a kind of present).
'on board' indicates putting letters inside.
'ship' becomes 'ss'.
'now' inserted into 'ss' is 'SNOWS'.
'for' is the link.
(Other definitions for snows that I've seen before include "Showers frozen flakes" , "Comes down in flakes" , "Precipitates" .)