Mislaid luggage containing uniform -- they really are useless (4,6)
I believe the answer is:
lost causes
'they really are useless' is the definition.
I don't know anything about this answer so I cannot tell whether it can be defined by this definition.
'mislaid luggage containing uniform' is the wordplay.
'mislaid' becomes 'lost' ('lose' can be a synonym of 'mislay').
'luggage' becomes 'cases' (I've seen this before).
'containing' is an insertion indicator.
'uniform' becomes 'U' (phonetic alphabet: alpha, bravo, charlie etc.).
'cases' enclosing 'u' is 'causes'.
'lost'+'causes'='LOST CAUSES'
Can you help me to learn more?
(Another definition for lost causes that I've seen is " people who can't succeed".)