Bow of flood rescue vessel in sound (3)
I believe the answer is:
arc
'bow' is the definition.
(both can mean a curve)
'flood rescue vessel in sound' is the wordplay.
'flood rescue vessel' becomes 'ark' (Noah's ark).
'in sound' shows a homophone (sound like).
'ark' sounds like 'ARC'.
'of' acts as a link.
(Other definitions for arc that I've seen before include "Bend" , "cut off from the circle" , "circle? Not entirely" , "be rapidly discharged" , "Section of circumference" .)