A copper holding middle of shotgun bolted down trigger (7)
I believe the answer is:
actuate
'trigger' is the definition.
(synonyms)
'a copper holding middle of shotgun bolted down' is the wordplay.
'copper' becomes 'Cu' (chemical symbol).
'holding' indicates putting letters inside.
'middle of' indicates the central letters.
'bolted down' becomes 'ate' (I've seen this in another clue).
The middle letter of 'shotgun' is 't'.
'cu' going around 't' is 'ctu'.
'a'+'ctu'+'ate'='ACTUATE'
(Other definitions for actuate that I've seen before include "Trigger off, energise" , "get going" , "Cause to operate" , "Put into action" , "trip?" .)