Calculating anaesthetic needed with crisis in middle of night (6-9)
I believe the answer is:
number-crunching
'calculating' is the definition.
(I've seen this before)
'anaesthetic needed with crisis in middle of night' is the wordplay.
'anaesthetic' becomes 'number' (i.e. something that makes things numb).
'needed with' is a charade indicator (letters next to each other).
'crisis' becomes 'crunch' (eg credit crunch).
'middle of night' becomes 'G' (middle letter of 'night').
'number'+'crunch'+'in'+'g'='NUMBER CRUNCHING'
(Other definitions for number-crunching that I've seen before include "Performing large numbers of calculations" , "Making lots of calculations" .)