Like an unchallenged receiver (4-4)
I believe the answer is:
wide-open
'like an unchallenged receiver' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for wide-open that I've seen before include "Gaping -- very vulnerable" , "Vulnerable to attack (colloq.)" , "Exposed to attack" .)