Open, due to start tomorrow (5)
I believe the answer is:
overt
'open' is the definition.
(synonyms)
'due to start tomorrow' is the wordplay.
'due to' becomes 'over' (as in quitting a job over an issue).
'start' suggests taking the first letters.
The initial letter of 'tomorrow' is 't'.
'over'+'t'='OVERT'
(Other definitions for overt that I've seen before include "Conspicuous" , "Plain to see - manifest" , "Plain and obvious" , "____ is able to be seen" , "Open and observable" .)