Practice in America: 'grey' oddly seen (5)
I believe the answer is:
usage
'practice' is the definition.
(usage is a kind of practice)
'america grey oddly seen' is the wordplay.
'america' becomes 'usa' (synonyms).
'oddly seen' indicates alternate letters.
The alternate letters of 'grey' are 'ge'.
'usa'+'ge'='USAGE'
'in' is the link.
(Other definitions for usage that I've seen before include "Accepted or habitual practice" , "Silly little thing" , "Customary way in which language is spoken or written" , "Convention" , "Treatment, practice" .)