Beefy yet big-headedly sheepish! (6)
I believe the answer is:
bovine
'beefy' is the definition.
Both the answer and definition are adjectives. Perhaps you can see an association between them that I can't see?
'big-headedly sheepish' is the wordplay.
'big-headedly' becomes 'b' (this might be a standard abbreviation which I don't know about).
'sheepish' becomes 'ovine' (I've seen this in another clue).
'b'+'ovine'='BOVINE'
'yet' is the link.
Can you help me to learn more?
(Other definitions for bovine that I've seen before include "dense" , "Resembling cattle" , "(Like a) yak, for example" , "Cowlike" , "Like a cow" .)