Most important muscle has too big a head! (8)

'most important muscle has too' is the definition.
I don't know anything about this answer so I cannot judge whether this works.

'big a head' is the wordplay.
I cannot quite see how this works, but
'big' could be 'heavy' (similar in meaning) and 'heavy' is found within the answer.
'head' could be 'top' (synonyms) and 'top' is found in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?