Unusual if Fred's treated by hospital department (9)
I believe the answer is:
different
'unusual' is the definition.
(similar in meaning)
'if fred's treated by hospital department' is the wordplay.
'treated' indicates anagramming the letters.
'by' says to put letters next to each other.
'hospital department' becomes 'ent'.
'if'+'fred'='iffred'
'iffred' is an anagram of 'differ'.
'differ'+'ent'='DIFFERENT'
(Other definitions for different that I've seen before include "Otherwise" , "Distinct, separate" , "Unusual" , "Dissimilar, unlike" .)