Bones cycling behind (6)
I believe the answer is:
sterna
'bones' is the definition.
(sternum is a kind of bone)
'cycling behind' is the wordplay.
I cannot quite see how this works, but
'behind' could be 'stern' (synonyms) and 'stern' is located in the answer.
A single letter 'a' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for sterna that I've seen before include "Breastbones" , "Bones in front" , "Formal backbones, Latin plural" .)