Struggling with this one. I know alien is normally ET and conservative is probably C or tory but can't figure out what the definition is. The letters I have so far are _ _ _ /O/_ I _ E / _ _ _ _ _ (Not entirely sure about the O either)
Your letters are correct, and the answer is:
CAT O'NINE TAILS
"Beating" is the definition here and the remainder of this clue is just a string of short words and abbreviations which form the phrase "CAT O'NINE TAILS", i.e. the multi-tailed whip used as a punishment, 'designed to lacerate the skin and cause intense pain' (Wikipedia).
As you suggest, 'conservative' gives us 'C' and we use the whole word 'AT' next. 'Working' is 'ON', then we have 'IN'. 'Alien' (again, as you rightly suggest) is 'E.T.', then 'area' gives us 'A'. We then have 'one' ('I' in Roman numerals), 'liberal' ('L') and 'success to start with' (i.e. the first letter of 'success', namely 'S').
Beating (clue) conservative (C) at (AT) working (ON) in (IN) alien (ET) area (A): one (I) liberal (L) success to start with (S) = CAT O'NINE TAILS.