Mechanical device in pit containing a chain (7)
I believe the answer is:
machine
'mechanical device' is the definition.
(I know that machine is a type of mechanical device)
'in pit containing a chain' is the wordplay.
'in pit' becomes 'mine' (mining pit. I am not sure about the 'in' bit.).
'containing' indicates putting letters inside.
'chain' becomes 'ch' (abbreviation for the unit of length).
'a'+'ch'='ach'
'mine' enclosing 'ach' is 'MACHINE'.
Can you help me to learn more?
(Other definitions for machine that I've seen before include "Well-organised and powerful group" , "Device providing energy to perform a task" , "Automaton" , "Industrial worker" , "Motor" .)