The boxer is a bit of a liability (3)
I believe the answer is:
ali
'the boxer' is the definition.
(Muhammad Ali)
'a bit of a liability' is the wordplay.
'bit of a liability' becomes 'li' (I can't justify this - if you can you should give a lot more credence to this answer).
'a'+'li'='ALI'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for ali that I've seen before include "girl shortly" , "Champion boxer" , "Lepidopterous floater, apian stinger, d. 2016" , "Muhammad ..., former boxer" , "Muhammad ... (former fighter)" .)