Pass out, seeing reptiles (4)
I believe the answer is:
asps
'reptiles' is the definition.
(asp is a kind of reptile)
'pass out' is the wordplay.
'out' is an anagram indicator (out can mean wrong or inaccurate).
'pass' is an anagram of 'ASPS'.
'seeing' acts as a link.
(Other definitions for asps that I've seen before include "people" , "Cleopatra's cobras" , "Poisonous creepers" , "Poplars" , "poisonous things" .)