Army admitting mistake for land (5)
I believe the answer is:
terra
'land' is the definition.
(I've seen this before)
'army admitting mistake' is the wordplay.
'army' becomes 'TA' (abbreviation for Territorial Army).
'admitting' means one lot of letters goes inside another (admit can mean to welcome in or accept).
'mistake' becomes 'err' (to err is to make a mistake or blunder).
'ta' going around 'err' is 'TERRA'.
'for' is the link.
(Other definitions for terra that I've seen before include "Latin word for earth" , "Centre" , "-- Firma; -- incognita" , "Land (Latin)" , "Dry land" .)