Famous school, observe, coming up (4)
I believe the answer is:
eton
'famous school' is the definition.
(I've seen this before)
'observe coming up' is the wordplay.
'observe' becomes 'note' ('note' can be a synonym of 'observe').
'coming up' says the letters should be written backwards.
'note' reversed gives 'ETON'.
(Other definitions for eton that I've seen before include "Town opposite Windsor on the Thames" , "Posh English school of note" , "Posh British public school" , "English college of note" , "Note sent up to English public school" .)