It's plain the pier is free to take the air (7)
I believe the answer is:
prairie
'it's plain' is the definition.
(type of plain)
'pier is free to take the air' is the wordplay.
'is free' indicates anagramming the letters.
'to take' indicates putting letters inside (inserted letters are taken in).
'pier' anagrammed gives 'prie'.
'prie' placed around 'air' is 'PRAIRIE'.
'the' is the link.
(Other definitions for prairie that I've seen before include "Broad rolling tract of land in the US" , "Tract of (US) grassland" , "Wide grassy plain" , "Grasslands" , "Large treeless tract of grassland" .)