A frontier row? (4)

I believe the answer is:

tier

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'a frontier row?' is the definition.
'tier' can be an answer for 'row?' (a tier is a row of things). I am not certain of the 'a frontier' bit.

'a frontier row?' is the wordplay.
'TIER' is hidden within the letters.
However, I'm unsure how this is indicated.

Can you help me to learn more?

(Other definitions for tier that I've seen before include "range" , "Echelon" , "Row, level" , "One of two or more rows" , "'Level, grade (4)'" .)

I've seen this clue in the Evening Standard.

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