As bottom cut short? (7)
I believe the answer is:
arsenic
'as' is the definition.
(As is the chemical symbol for arsenic)
'bottom cut short?' is the wordplay.
'bottom' becomes 'arse' (synonyms).
'cut' becomes 'nick' (nicking is a kind of cutting).
'short?' means to remove the last letter (I've seen 'short' mean this).
'nick' with its last letter taken away is 'nic'.
'arse'+'nic'='ARSENIC'
(Other definitions for arsenic that I've seen before include "Element No 33" , "As briefly identified" , "Toxic metallic element used as rat poison" , "Brittle grey-green semi-metal, atomic number 33" , "Ratsbane" .)