Really short bum established agency (8)
I believe the answer is:
umbrella
'agency' is the definition.
(I have seen 'Overall agency' mean 'umbrella' so perhaps 'agency' could also mean 'umbrella')
'really short bum established' is the wordplay.
I cannot really see how this works, but
an anagram of 'bum' is 'umb' which is present in the answer.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for umbrella that I've seen before include "It keeps the raini off" , "Rain shield" , "cover for the British weather?" , "The old song urges to let a smile be yours" , "Brolly" .)