You do it or even America will become extremely unpleasant (6)
I believe the answer is:
odious
'extremely unpleasant' is the definition.
(I've seen this before)
'you do it or even america' is the wordplay.
'even' indicates alternate letters (even-numbered letters are used).
'america' becomes 'us' (abbreviation for the United States).
'you'+'do'+'it'+'or'='youdoitor'
The alternating letters of 'youdoitor' are 'odio'.
'odio'+'us'='ODIOUS'
'will become' is the link.
(Other definitions for odious that I've seen before include "Unpleasant, repulsive" , "Abominable, detestable" , "Really hateful, abhorrent" , "Thoroughly obnoxious, hateful" , "Repellent" .)