Literary giant ruined pursuing legal action (8)
I believe the answer is:
litigant
'pursuing legal action' is the definition.
(involved in litigation)
'literary giant ruined' is the wordplay.
'literary' becomes 'lit' (short for literary or literature).
'ruined' indicates anagramming the letters.
'giant' is an anagram of 'igant'.
'lit'+'igant'='LITIGANT'
(Other definitions for litigant that I've seen before include "Man in suit" , "One going to law" , "One who takes a lawsuit" , "Plaintiff" , "Person involved in a lawsuit" .)