Expression of surprise on encountering a feature of eclipse (6)
I believe the answer is:
corona
'feature of eclipse' is the definition.
(disc of gas visible during an eclipse)
'expression of surprise on encountering a' is the wordplay.
'expression of surprise' becomes 'cor' ('cor!' is an expression of surprise).
'encountering' means one lot of letters go next to another.
'cor'+'on'+'a'='CORONA'
(Other definitions for corona that I've seen before include "Glow or circle of light" , "Ring round the sun, or a big cigar" , "Racoon (anag.)" , "Big Smoke" , "Sun's gaseous envelope" .)