Commanding officer with case for detective Morse, say (4)
I believe the answer is:
code
'morse say' is the definition.
(morse is a kind of code)
'commanding officer with case for detective' is the wordplay.
'commanding officer' becomes 'co'.
'with' is a charade indicator (letters next to each other).
'case for' says to hollow out the word (remove centre letters) (the letters which encase the others are used).
'detective' with its centre taken out is 'de'.
'co'+'de'='CODE'
(Other definitions for code that I've seen before include "System of words arrangement for secrecy" , "Collection of laws" , "Encryption, disguised language" , "It may be impossible to read" , "See 21 Across" .)