Cube; rowing boat (5)
I believe the answer is:
eight
I believe this clue is a double definition.
'cube' is the first definition.
(8 is a cube number)
'rowing boat' is the second definition.
(I've seen this before)
(Other definitions for eight that I've seen before include "Oxygen's atomic number" , "team rowing" , "Figure" , "Crew of racing shell" , "power of two" .)