Understanding number in part of hospital will require particular drug (7)
I believe the answer is:
entente
'understanding' is the definition.
(an entente is an alliance or understanding)
'number in part of hospital will require particular drug' is the wordplay.
'number' becomes 'ten' (ten is a kind of whole number).
'in' indicates putting letters inside.
'part of hospital' becomes 'ent' (abbreviation for Ear, Nose and Throat department).
'will require' says to put letters next to each other.
'particular drug' becomes 'E' (an ecstasy tablet).
'ten' put into 'ent' is 'entent'.
'entent'+'e'='ENTENTE'
(Other definitions for entente that I've seen before include "Interstate understanding" , "International agreement" , "Friendly arrangement" , "Friendly understanding between states" , "Amicable agreement" .)