Oh, no! Overturned mercury runs all around initially (5)
I believe the answer is:
aargh
'oh no' is the definition.
(expression of horror or dismay)
'overturned mercury runs all around initially' is the wordplay.
'overturned' shows that the letters should be reversed in order.
'mercury' becomes 'hg' (Hg is the chemical symbol for mercury).
'initially' indicates taking the first letters.
The first letters of 'runs all around' is 'raa'.
'hg'+'raa'='hgraa'
'hgraa' written backwards gives 'AARGH'.
(Other definitions for aargh that I've seen before include "Exclamation of frustration or rage" , "Cry of utter frustration" , "I'm so frustrated!" , "I'm so exasperated!" , "that's dreadful!" .)