Before echo sounders, so lost on a submarine (8)
I believe the answer is:
undersea
'submarine' is the definition.
(synonyms)
'before echo sounders so lost on a' is the wordplay.
'before' means one lot of letters go next to another.
'echo' becomes 'e' (phonetic alphabet: alpha, bravo, charlie etc.).
'lost' is a deletion indicator.
'on' says to put letters next to each other.
'sounders' with 'so' removed is 'unders'.
'e' after 'unders' is 'underse'.
'underse'+'a'='UNDERSEA'
(Other definitions for undersea that I've seen before include "Beneath the surface of the ocean" , "submerged" , "Where bed is" , "Beneath the oceans" , "in salt water" .)