Rush on an extremely risky medical intervention (7)
I believe the answer is:
surgery
'medical intervention' is the definition.
(I've seen this before)
'rush on an extremely risky' is the wordplay.
'rush' becomes 'surge' (synonyms).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'an extremely' says to hollow out the word (remove centre letters) (only the extremes of the word are used).
'risky' with its middle taken out is 'ry'.
'surge'+'ry'='SURGERY'
(Other definitions for surgery that I've seen before include "Doctors' practice" , "Doctor's consulting room" , "Doctor's premises" , "NHS cuts" , "Branch of medicine" .)