Return of illness making royal pass by (7)
I believe the answer is:
relapse
'return of illness' is the definition.
(I've seen this before)
'royal pass by' is the wordplay.
'royal' becomes 'r' (common abbreviation e.g. RM - Royal Marine).
'pass by' becomes 'elapse' (to elapse is to run out or pass by).
'r'+'elapse'='RELAPSE'
'making' is the link.
(Other definitions for relapse that I've seen before include "Become ill again" , "Fall back into illness" , "Get worse again" , "Decline" , "Fall back (into ill health)" .)