Open organisation starts letting anyone take casual hours (7)
I believe the answer is:
unlatch
'open' is the definition.
(unlatching something is opening it)
'organisation starts letting anyone take casual hours' is the wordplay.
'organisation' becomes 'un' (the United Nations is an organisation).
'starts' says to take the initial letters.
The first letters of 'letting anyone take casual hours' is 'latch'.
'un'+'latch'='UNLATCH'
(Other definitions for unlatch that I've seen before include "Unfasten (a door)" , "Free bar" .)