As I must be among mobile cranes (7)
I believe the answer is:
arsenic
'as' is the definition.
(As is the chemical symbol for arsenic)
'i must be among mobile cranes' is the wordplay.
'must be among' is an insertion indicator.
'mobile' is an anagram indicator (mobile can mean moving).
'cranes' with letters rearranged gives 'arsenc'.
'i' inserted within 'arsenc' is 'ARSENIC'.
(Other definitions for arsenic that I've seen before include "Brittle grey-green semi-metal, atomic number 33" , "Powerful poison" , "Can rise result from poison?" , "Toxic metallic element used as rat poison" , "Ratsbane" .)