Probe trustee about rank concealing moral quality (4,3,5)
I believe the answer is:
test the water
'probe trustee about rank concealing moral quality' is the definition.
I don't know anything about this answer so I can't judge whether this works.
'probe trustee about rank concealing moral quality' is the wordplay.
'probe' becomes 'test' (I've seen this in another clue).
'trustee' becomes 't' (abbreviation).
'about rank concealing moral' becomes 'he' (I can't justify this - if you can you should believe this answer much more).
'quality' becomes 'water' (eg the phrase 'finest water' meaning high quality).
'test'+'t'+'he'+'water'='TEST THE WATER'
Can you help me to learn more?
(Other definitions for test the water that I've seen before include "Seek opinions before proceeding" , "Take soundings" , "Seek to identify a probable reaction" , "Sound out reactions" , "Sound out before taking further action" .)