Hero's cross, mostly open up-front (4)
I believe the answer is:
ajax
'hero's' is the definition.
(I've seen this before)
'cross mostly open up-front' is the wordplay.
'cross' becomes 'x' (X is cross-shaped).
'mostly open' becomes 'aja' (I can't justify this - if you can you should believe this answer much more).
'up front' says to put letters next to each other.
'x' after 'aja' is 'AJAX'.
Can you help me to learn more?
(Other definitions for ajax that I've seen before include "Swiftest Greek warrior in the Trojan War who killed himself" , "side" , "Greek hero in The Iliad" , "Cleaner" , "Greek hero of the Trojan war - Dutch football team" .)