Daisy, you'd look sweet sitting on one (6)
I believe the answer is:
tandem
'daisy you'd' is the definition.
The definition suggests a singular noun which matches the answer.
'look sweet sitting on one' is the wordplay.
I cannot really see how this works, but
'one' could be 'an' (an apple is one apple) and 'an' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for tandem that I've seen before include "Bicycle for girl in 32 Across" , "Bike with two saddles" , "Two-seat bicycle" , "vehicle" , "Two-wheeler for two" .)