One's getting on in x-ray department (7)
I believe the answer is:
imaging
'x-ray department' is the definition.
'one's getting on' is the wordplay.
'one's' becomes 'im' ('one is' is a formal way of saying 'I am').
'getting on' becomes 'aging' (I've seen this before).
'im'+'aging'='IMAGING'
'in' is the link.
(Other definitions for imaging that I've seen before include "Forming a picture of (something)" , "Making pictorial representation of (something)" , "Forming a mental likeness of (something)" , "Making a picture" , "Making pictures" .)