Thought to do in with arsenic strangely (13)
I believe the answer is:
consideration
'thought' is the definition.
(I've seen this before)
'to do in with arsenic strangely' is the wordplay.
'with' says to put letters next to each other.
'strangely' is an anagram indicator.
'to'+'do'+'in'+'arsenic'='todoinarsenic'
'todoinarsenic' with letters rearranged gives 'CONSIDERATION'.
(Other definitions for consideration that I've seen before include "Reward; careful thought" , "Kindness" , "Thoughtfulness - relevant factor" , "A motive" , "Bearing in mind - payment" .)