Everyone with long legs perhaps striking head (3)
I believe the answer is:
all
'everyone' is the definition.
(all people)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for all that I've seen before include "Entire" , "Long avenue or garden path" , "Very nearly" , "nothing more" , "Passage in a formal garden" .)