As a river starts to surge, everything nearby is consumed (7)
I believe the answer is:
arsenic
'as' is the definition.
(As is the chemical symbol for arsenic)
'a river starts to surge everything nearby is consumed' is the wordplay.
'river' becomes 'r' (abbreviation).
'starts to' indicates taking the first letters.
The first letters of 'surge everything nearby is consumed' is 'senic'.
'a'+'r'+'senic'='ARSENIC'
(Other definitions for arsenic that I've seen before include "Famous stage comedy ''. . . . . . . and Old Lace''" , "Ratsbane" , "Can rise result from poison?" , "Poison used in pesticides and weedkillers" , "Brittle grey-green semi-metal, atomic number 33" .)