One thousand and three and a bit, a Zulu unit (4)

I believe the answer is:

impi

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'a zulu unit' is the definition.
The definition suggests a singular noun which matches the answer. I also know that 'zulu' relates to this answer.

'one thousand and three and a bit' is the wordplay.
'one' becomes 'i' (Roman numeral).
'thousand' becomes 'm' (Roman numeral for a thousand).
'and' means one lot of letters go next to another.
'three and a bit' becomes 'pi' (3.141592...).
'i'+'m'+'pi'='IMPI'

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(Other definitions for impi that I've seen before include "group of soldiers" , "Fighting force" , "Old warriors" , "Group of Bantu warriors" , "native warriors?" .)

I've seen this clue in The Independent.

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