Clue: Help revolutionary over variable period in US (8)
Answer: DYSTOPIA
Explanation: Definition: US
An envelope ('over') of Y (mathematical 'variable') plus STOP (full, 'period') in DIA, a reversal ('revolutionary') of AID ('help'). I take it that the "definition" is a comment on the recent state of affairs in the United States, which would make it an unannounced indication by example. U/S for unnserviceable is the wrong part of speech. *See Crucible's comment @22.