Attempt at goal goes around post from one of those not in form but is over those who are (10)

'attempt at goal goes around post from one of those not in form but is over those who are' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.

'form but is over those who are' is the wordplay.
I cannot quite see how this works, but
'form' could be 'head' (head is a kind of form) and 'head' is found in the answer.
'are' could be 'a' (short for 'are', historical unit of measurement) and 'a' is present in the answer.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?