Pass by post room beginning to deteriorate (7)
I believe the answer is:
relapse
'to deteriorate' is the definition.
(as in a relapse of a medical condition)
'pass by post room beginning' is the wordplay.
'pass by' becomes 'elapse' (to elapse is to run out or pass by).
'post' says to put letters next to each other (post- can mean after).
'beginning' indicates taking the first letters.
The initial letter of 'room' is 'r'.
'elapse' put after 'r' is 'RELAPSE'.
(Other definitions for relapse that I've seen before include "Become ill again" , "medical problem" , "Fall back into former condition, being pleaser" , "Deteriorate after getting better" , "Downturn in health (after improvement)" .)