Dark one. Fiend. I have a different name? (10)
I believe the answer is:
diminutive
'a different name?' is the definition.
'dark one fiend i have' is the wordplay.
'dark' becomes 'dim' ('dim' can be similar in meaning to 'dark').
'one' becomes 'i' (Roman numeral).
'fiend' becomes 'nut' (both can mean someone obsessed with something).
'i have' becomes 'ive'.
'dim'+'i'+'nut'+'ive'='DIMINUTIVE'
(Other definitions for diminutive that I've seen before include "Pocket-sized" , "For example Billy" , "Tiny" , "Extremely small" , "Undersized" .)