To doctors they're grave situations (5)
I believe the answer is:
tombs
'grave situations' is the definition.
Although both the answer and definition are plural nouns, I don't see how one could define the other.
'to doctors' is the wordplay.
'doctors' becomes 'MBs' (Bachelors of Medicine).
'to'+'mbs'='TOMBS'
'they're' acts as a link.
Can you help me to learn more?
(Other definitions for tombs that I've seen before include "Evidence of failure" , "monuments" , "Sepulchres" , "Places of burial" .)