Son on edge — one of those in rescue vessel? (4)
I believe the answer is:
shem
'one of those in rescue vessel?' is the definition.
(son of Noah)
'son on edge' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'on' is a charade indicator (letters next to each other).
'edge' becomes 'hem' (I've seen this before).
's'+'hem'='SHEM'
(Other definitions for shem that I've seen before include "Biblical character" , "Eldest of Noah's three sons" , "Noah's son caught in mess" , "One of Noah's sons" , "Third son of Noah" .)