Horse: lad astride big one (8)
I believe the answer is:
stallion
'horse' is the definition.
(I know that stallion is a type of horse)
'lad astride big one' is the wordplay.
'lad' becomes 'son' (lad can informally mean a young boy).
'astride' means one lot of letters goes inside another.
'big' becomes 'tall'.
'one' becomes 'i' (Roman numeral).
'tall'+'i'='talli'
'son' going around 'talli' is 'STALLION'.
(Other definitions for stallion that I've seen before include "Male horse, often used for breeding" , "Male steed" , "Uncastrated adult male horse" , "Male horse kept for breeding" , "Mare's mate" .)