Is rattling on about high horse (8)
I believe the answer is:
stallion
'horse' is the definition.
(I know that stallion is a type of male horse)
'is rattling on about high' is the wordplay.
'rattling' indicates an anagram (letters rattle around).
'about' means one lot of letters goes inside another.
'high' becomes 'tall' (similar in meaning).
'is' anagrammed gives 'si'.
'si'+'on'='sion'
'sion' enclosing 'tall' is 'STALLION'.
(Other definitions for stallion that I've seen before include "Male horse, often used for breeding" , "Stud" , "Male steed" , "Male horse lost in atolls" , "Mount" .)