Measurer of depth, thus submerged by a nymph (4,7)
I believe the answer is:
echo sounder
'measurer of depth' is the definition.
'thus submerged by a nymph' is the wordplay.
'thus' becomes 'so' ('so' can be a synonym of 'thus').
'submerged' becomes 'under'.
'by' means one lot of letters go next to another.
'a nymph' becomes 'echo' (figure of Greek myth).
'so'+'under'='sounder'
'sounder' after 'echo' is 'ECHO-SOUNDER'.
(Other definitions for echo sounder that I've seen before include "Sea depth measurer" , "Means of determining depth at sea" , "Sonar" , "Device that can map seabed" , "used to hit rock-bottom" .)