As nice as rocks, seen around river (7)
I believe the answer is:
arsenic
'as' is the definition.
(As is the chemical symbol for arsenic)
'nice as rocks seen around river' is the wordplay.
'rocks' is an anagram indicator.
'seen around' means one lot of letters goes inside another.
'river' becomes 'r' (abbreviation for river).
'nice'+'as'='niceas'
'niceas' with letters rearranged gives 'asenic'.
'asenic' going around 'r' is 'ARSENIC'.
(Other definitions for arsenic that I've seen before include "killer" , "Toxic metallic element used as rat poison" , "Poisons in scare" , "Poison used in pesticides and weedkillers" , "Powerful poison" .)