Vet's demeanour strangely enterprising (13)
I believe the answer is:
adventuresome
'enterprising' is the definition.
(associated in meaning)
'vet's demeanour strangely' is the wordplay.
'strangely' indicates an anagram.
'vets'+'demeanour'='vetsdemeanour'
'vetsdemeanour' anagrammed gives 'ADVENTURESOME'.
(Other definitions for adventuresome that I've seen before include "taking risks" , "Ready to take risks" , "Ready to incur risk" , "Daring and enterprising" , "Rather daring arrival" .)