Saving hospital doctor saline's the breath of life (9)
I believe the answer is:
essential
'breath of life' is the definition.
(the breath of life means something essential)
'saving hospital doctor saline's the' is the wordplay.
'saving hospital' becomes 'es' (I can't explain this - if you can you should believe this answer much more).
'doctor' is an anagram indicator.
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'saline'+'t'='salinet'
'salinet' is an anagram of 'sential'.
'es'+'sential'='ESSENTIAL'
(Other definitions for essential that I've seen before include "'Absolutely necessary, required (9)'" , "must-have item" , "something necessary" , "Indispensible" , "Basic or necessary" .)