Crossword Genius

Some Zulus travel for five year periods (6)

Ross

I believe the answer is:

lustra

I'm a little stuck... Click here to teach me more about this clue!

'five year periods' is the definition.
Although both the answer and definition are plural nouns, I cannot understand how they can define each other.

'some zulus travel for' is the wordplay.
'some' says the answer is hidden in the clue (some parts of the word are used).
'LUSTRA' can be found hidden inside 'zulus travel for'.

Can you help me to learn more?

(Other definitions for lustra that I've seen before include "Periods of five years" , "Five-year periods" , "five year term" , "At least ten years" .)

I've seen this clue in The Independent.
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