Shot taken when you’re definitely not at your best (4,2,3,3)
I believe the answer is:
hair of the dog
'shot' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'when you're definitely not at your best' is the wordplay.
I cannot really see how this works, but
'when' could be 'e' (I've seen this in other clues) and 'e' is present in the answer.
'not' could be 'o' (I've seen this in other clues) and 'o' is found within the answer.
'best' could be 'ai' (resembles 'A1', old-fashioned term for excellent) and 'ai' is found within the answer.
This explanation may well be incorrect...
'taken' is the link.
Can you help me to learn more?
(Other definitions for hair of the dog that I've seen before include "For example, setter's warmer" , "breakfast-time drink?" , "Morning tipple after night of excessive drinking" , "Maybe one in the morning" , "A morning-after drink?" .)