Cold hospital department radiographer's opening with a large key (7)
I believe the answer is:
central
'key' is the definition.
(synonyms)
'cold hospital department radiographer's opening with a large' is the wordplay.
'cold' becomes 'c' (eg on taps).
'hospital department' becomes 'ENT' (abbreviation for Ear, Nose and Throat department).
'opening' says to take the initial letters.
'with' is a charade indicator (letters next to each other).
'large' becomes 'L' (eg in clothes sizes).
The first letter of 'radiographers' is 'r'.
'c'+'ent'+'r'+'a'+'l'='CENTRAL'
(Other definitions for central that I've seen before include "Most important, principal" , "Every answer here has one such letter" , "Focal" , "Key" , "Near the middle" .)