British don’t have to fight — that need not be understood (5,3)
I believe the answer is:
black box
'british don't have to fight that need not be understood' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
'british don't have to fight' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in BBC).
'don't have' becomes 'lack' (I've seen this in another clue).
'to fight' becomes 'box' (boxing is a kind of fighting).
'b'+'lack'+'box'='BLACK BOX'
Can you help me to learn more?
(Other definitions for black box that I've seen before include "item revealing how Jumbo killed people?" , "Aircraft's flight recorder" , "It contains information about an air crash" , "Flight data recorder" , "Electronic device in aircraft recording flight information" .)