At the summit, apt to get upset about nothing (4)
I believe the answer is:
atop
'at the summit' is the definition.
(I've seen this before)
'apt to get upset about nothing' is the wordplay.
'to get upset' indicates an anagram (I've seen 'getting upset' mean this).
'about' is an insertion indicator.
'nothing' becomes 'O' (looks like zero - 0).
'apt' with letters rearranged gives 'atp'.
'atp' enclosing 'o' is 'ATOP'.
(Other definitions for atop that I've seen before include "highly placed" , "Covering" , "Situated on" , "Positioned on" , "Cooker heating element" .)