Character supports strike (9)
I believe the answer is:
backslash
'character' is the definition.
(type of symbol)
'supports strike' is the wordplay.
'supports' becomes 'backs' (backing is a kind of supporting).
'strike' becomes 'lash' (lashing is a kind of striking).
'backs'+'lash'='BACKSLASH'
(Other definitions for backslash that I've seen before include "Mark" , "character" , "//www.crosswordgiant.com/crossword-clue/1252911/" , "One leaning to the left" , "line on computer screen" .)