Gang in uniform head regularly boxed before assembly? (4-6)

I believe the answer is:

flat-packed

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'boxed before assembly?' is the definition.
I can't tell whether this definition defines the answer.

'gang in uniform head regularly' is the wordplay.
'gang' becomes 'pack' (synonyms).
'in' is an insertion indicator.
'uniform' becomes 'flat' (I've seen this in other clues).
'head regularly' becomes 'ed' (I can't explain this - if you can you should believe this answer much more).
'flat'+'ed'='flated'
'pack' going into 'flated' is 'FLAT-PACKED'.

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