In Glamorgan I'm a lumbering beast (6)
I believe the answer is:
animal
'beast' is the definition.
(a beast is an animal)
'in glamorgan i'm a lumbering' is the wordplay.
'in' indicates a hidden word.
'ANIMAL' is hidden in the letters of 'glamorgan im a lumbering'.
(Other definitions for animal that I've seen before include "Circus performer, maybe" , "Brutish sort" , "Brutal, sensual" , "' - - Farm', Orwell" , "Kind of instinct" .)