Whole of hospital department having extremely deficient siren (6)
I believe the answer is:
entire
'whole' is the definition.
(similar in meaning)
'hospital department having extremely deficient siren' is the wordplay.
'hospital department' becomes 'ENT' (Ear, Nose and Throat department).
'having' is a charade indicator (letters next to each other).
'extremely deficient' says to take the centre.
The middle of 'siren' is 'ire'.
'ent'+'ire'='ENTIRE'
'of' is the link.
(Other definitions for entire that I've seen before include "excluding nothing" , "Utter" , "Whole and complete" , "With no part left out" , "Full and complete" .)