Disqualified one active in e.g. libel (10)
I believe the answer is:
ineligible
'disqualified' is the definition.
(similar in meaning)
'one active in e g libel' is the wordplay.
'one' becomes 'i' (Roman numeral).
'active' indicates an anagram (the letters are active/moving).
'in'+'e'+'g'+'libel'='ineglibel'
'ineglibel' is an anagram of 'neligible'.
'i'+'neligible'='INELIGIBLE'
(Other definitions for ineligible that I've seen before include "Not qualified - lie in bilge (anag)" , "Not qualified, unsuitable" .)