Cunning mum ruined late husband (7)
I believe the answer is:
stealth
'cunning' is the definition.
(thesaurus)
'mum ruined late husband' is the wordplay.
'mum' becomes 'st' (I am not sure about this - if you are sure you should believe this answer much more).
'ruined' indicates anagramming the letters.
'husband' becomes 'h' (abbreviation).
'late' with letters rearranged gives 'ealt'.
'st'+'ealt'+'h'='STEALTH'
Can you help me to learn more?
(Other definitions for stealth that I've seen before include "Kind of indirect tax" , "underhand dealing" , "Low-visibility military technology" , "quality needed by cat burglars" , "Slyness" .)