Superhuman boy leaves a Greek order (5)
I believe the answer is:
ionic
'a greek order' is the definition.
(I've seen this before)
'superhuman boy leaves' is the wordplay.
'superhuman' becomes 'bionic' (I've seen this before).
'boy' becomes 'b' (abbreviation used in schools).
'leaves' suggests deleting specific letters.
'bionic' with 'b' taken away is 'IONIC'.
(Other definitions for ionic that I've seen before include "entailing small charges" , "Relating to charged particles" , "Sort of column" , "Like some Greek architecture" , "One of five classical orders of architecture" .)