Tandem bike's capacity (3)
I believe the answer is:
two
'tandem bike's capacity' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for two that I've seen before include "II" , "Minimal number of partners" , "Prime number" , "Old song 'Tea for . . .'" , "Company, so it's said" .)