A doctor who's sacked must be so rusty (3,2,8)
I believe the answer is:
out of practice
'rusty' is the definition.
'a doctor who's sacked must be' is the wordplay.
I cannot quite see how this works, but
'a' is within the answer.
'sacked' could be 'out' and 'out' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
'so' acts as a link.
Can you help me to learn more?
(Other definitions for out of practice that I've seen before include "being rusty?" , "Like a doctor dismissed" .)