Old article about one hospital department (7)
I believe the answer is:
ancient
'old' is the definition.
('ancient' can be similar in meaning to 'old')
'article about one hospital department' is the wordplay.
'article' becomes 'an' (indefinite article).
'about' becomes 'c' (abbreviation for 'circa'**).
'one' becomes 'i' (Roman numeral).
'hospital department' becomes 'ent'.
'an'+'c'+'i'+'ent'='ANCIENT'
(Other definitions for ancient that I've seen before include "Dating from long ago" , "Aged and timeworn" , "Bygone" , "from olden times" , "of long standing" .)