All sit on rocking horse (8)
I believe the answer is:
stallion
'horse' is the definition.
(I know that stallion is a type of horse)
'all sit on rocking' is the wordplay.
'rocking' indicates an anagram.
'all'+'sit'+'on'='allsiton'
'allsiton' is an anagram of 'STALLION'.
(Other definitions for stallion that I've seen before include "Mare's mate" , "Male horse, often used for breeding" , "Uncastrated adult male horse" , "Mount" , "Male horse kept for breeding" .)