Invitation postponed when sewer inspection fails to start (4,5)
I believe the answer is:
rain check
'invitation postponed' is the definition.
'sewer inspection fails to start' is the wordplay.
'sewer' becomes 'drain' (I've seen this before).
'inspection' becomes 'check' (check is a kind of inspection).
'fails to start' means to remove the first letter.
'drain'+'check'='draincheck'
'draincheck' with its initial letter removed is 'RAIN CHECK'.
'when' acts as a link.
(Other definitions for rain check that I've seen before include "Postponement" , "Ticket issued for later use if an event is delayed through weather (US)" , "Deferral of acceptance - testing weather?" .)