A certain sound that's out of the blue? (6)
I believe the answer is:
ashore
'out of the blue?' is the definition.
('blue' can mean the sea)
'a certain sound' is the wordplay.
'certain' becomes 'sure' ('sure' can be a synonym of 'certain').
'sound' indicates a 'sounds like' (homophone) clue.
'sure' sounds like 'shore'.
'a'+'shore'='ASHORE'
'that's' acts as a link.
(Other definitions for ashore that I've seen before include "Going on land from sea" , "beach" , "on the coast" , "in port?" , "Onto land (from the water)" .)