Eccentric was old assassin in Dallas (6)
I believe the answer is:
oswald
'assassin in dallas' is the definition.
'oswald' can be an answer for 'assassin' (I've seen this before). I'm not certain of the 'in dallas' bit.
'eccentric was old' is the wordplay.
'eccentric' indicates anagramming the letters.
'was'+'old'='wasold'
'wasold' is an anagram of 'OSWALD'.
Can you help me to learn more?
(Other definitions for oswald that I've seen before include "Lee Harvey . . . . . . assassinated President Kennedy" , "Man" , "Lee Harvey ---, president Kennedy's killer" , "Lee Harvey . . . . . . assassinated J F Kennedy" , "King of the Northumbrians, d. 642 - President Kennedy's assassin, d. 1963" .)