Stands in boats (4)
I believe the answer is:
subs
This could be a double definition.
'stands in' is the first definition.
('sub' can be a synonym of 'stand in')
'boats' is the second definition.
The definition and answer can be both man-made objects as well as being plural nouns.
Perhaps they are linked in a way I don't understand?
Can you help me to learn more?
(Other definitions for subs that I've seen before include "Club members' payments" , "vehicles" , "Advance payments" , "Reserves at football (sl.)" , "Players in reserve" .)