Double-cross (6)
I believe the answer is:
betray
'double-cross' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for betray that I've seen before include "26 [SELL]" , "Double-cross, sell out" , "Sell down the river" , "Reveal (intentionally or unintentionally)" , "Breach the trust of" .)