As nice cycling as taking runs to begin with (7)
I believe the answer is:
arsenic
'as' is the definition.
(As is the chemical symbol for arsenic)
'nice cycling as taking runs to begin with' is the wordplay.
'cycling' indicates an anagram.
'taking' is an insertion indicator.
'runs' becomes 'r' (cricket abbreviation).
'to begin with' says to put letters next to each other.
'as' going around 'r' is 'ars'.
'nice' is an anagram of 'enic'.
'enic' after 'ars' is 'ARSENIC'.
(Other definitions for arsenic that I've seen before include "Element with many poisonous compounds" , "Toxic metallic element used as rat poison" , "Poison found in races" , "Poison used in pesticides and weedkillers" , "Ratsbane" .)