Amusing the eccentric lord Lucan, initially (5)
I believe the answer is:
droll
'amusing' is the definition.
(I've seen this in another clue)
'eccentric lord lucan initially' is the wordplay.
'eccentric' is an anagram indicator.
'initially' indicates taking the first letters.
The first letter of 'lucan' is 'l'.
'lord' anagrammed gives 'drol'.
'drol'+'l'='DROLL'
'the' acts as a link.
(Other definitions for droll that I've seen before include "Comical in an odd or whimsical manner" , "Large amount (of matter)" , "Drily amusing" , "Drily witty" , "Strangely amusing" .)