A rescue organised outside very quiet capital (5-4)
I believe the answer is:
upper-case
'capital' is the definition.
('upper case' can be a synonym of 'capital')
'a rescue organised outside very quiet' is the wordplay.
'organised' is an anagram indicator.
'outside' indicates putting letters inside.
'very quiet' becomes 'pp'.
'a'+'rescue'='arescue'
'arescue' with letters rearranged gives 'uercase'.
'uercase' going around 'pp' is 'UPPER-CASE'.
(Other definitions for upper-case that I've seen before include "Capital" , "In capitals" .)