Tries to reform the Post Office inside to get a quick answer (7)
I believe the answer is:
riposte
'a quick answer' is the definition.
(I've seen this before)
'tries to reform the post office inside' is the wordplay.
'to reform' indicates an anagram (letters in a new form).
'the post office' becomes 'po' (abbreviation).
'inside' means one lot of letters goes inside another.
'tries' anagrammed gives 'riste'.
'riste' placed around 'po' is 'RIPOSTE'.
'to get' acts as a link.
(Other definitions for riposte that I've seen before include "Reply, retort" , "Rejoinder" , "Witty comeback" , "Smart reply" , "Counter-thrust" .)