He or she so unlikely to bring good luck (9)
I believe the answer is:
horseshoe
'good luck' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'he or she so unlikely' is the wordplay.
'unlikely' indicates an anagram.
'he'+'or'+'she'+'so'='heorsheso'
'heorsheso' is an anagram of 'HORSESHOE'.
'to bring' acts as a link.
Can you help me to learn more?
(Other definitions for horseshoe that I've seen before include "Bat" , "Iron plate beneath a hoof" , "Shore hose (anag.)" , "Lucky sign" , "Hoof protector" .)