Cycling laden, runs into marsh creature (8)
I believe the answer is:
bullfrog
'cycling laden runs' is the definition.
I can't tell whether this definition defines the answer.
'marsh creature' is the wordplay.
'marsh' becomes 'bull' (I can't explain this - if you can you should give a lot more credence to this answer).
'creature' becomes 'frog' (frog is a kind of creature).
'bull'+'frog'='BULLFROG'
'into' is the link.
Can you help me to learn more?
(Other definitions for bullfrog that I've seen before include "Large croaker" , "Large leaping amphibian" , "Jumper" , "Amphibian with deep croak" , "Creature with a deep croak" .)