A four-foot cycle (6)
I believe the answer is:
tandem
'cycle' is the definition.
(I know that tandem is a type of cycle)
'a four-foot' is the wordplay.
I cannot quite see how this works, but
'a' could be 'an' and 'an' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for tandem that I've seen before include "vehicle" , "Bicycle for two riders" , "Two wheels for two" , "Two-person bike" , "Bicycle built for two" .)