Big, strong boofhead spoke ahead of time (5)
I believe the answer is:
burly
'big' is the definition.
(thesaurus)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for burly that I've seen before include "Muscular and heavily built" , "Big and thickset" , "Beefy" , "Big and sturdy" , "Sturdily built" .)