Captain has anaesthetic unit (6,3)
I believe the answer is:
number one
'captain' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'anaesthetic unit' is the wordplay.
'anaesthetic' becomes 'number' (i.e. something that makes things numb).
'unit' becomes 'one' (synonyms).
'number'+'one'='NUMBER ONE'
'has' acts as a link.
Can you help me to learn more?
(Other definitions for number one that I've seen before include "top of the pops" , "top-selling record" , "arch" , "top man?" , "Informally and numerically, oneself" .)