Crossword Genius

Frontier line? (4)

Ross

I believe the answer is:

tier

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

'frontier line?' is the definition.
'tier' can be an answer for 'line?' (tier is a kind of line). I am unsure of the 'frontier' bit.

'frontier line?' is the wordplay.
'TIER' is hidden within the letters.
However, I am uncertain how this is indicated.

Can you help me to learn more?

(Other definitions for tier that I've seen before include "Row (of seats)" , "Person joining" , "Rite about layer" , "I might draw" , "Row, level" .)

I've seen this clue in the Evening Standard.
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