Sucker riding small brown horse (7)
I believe the answer is:
mustang
'sucker' is the definition.
The definition and answer can be both animals as well as being singular nouns.
Perhaps there's an association between them I don't understand?
'riding small brown horse' is the wordplay.
'riding' indicates putting letters inside.
'small' becomes 's' (abbreviation - e.g. clothes size).
'brown' becomes 'tan' (tan is a kind of brown).
'horse' becomes 'mug' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
's'+'tan'='stan'
'stan' put within 'mug' is 'MUSTANG'.
Can you help me to learn more?
(Other definitions for mustang that I've seen before include "Small, wild US horse" , "creature" , "one running wild" , "Small feral horse of the US" , "Wild horse of the West" .)