A frontier row? (4)

I believe the answer is:

tier

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

'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 in amongst the letters.
However, I'm not sure how the hidden word 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.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.