Copper on trial is composed in dock (7)
I believe the answer is:
curtail
'dock' is the definition.
(I've seen this in another clue)
'copper on trial is composed' is the wordplay.
'copper' becomes 'cu' (chemical symbol).
'on' is an insertion indicator.
'is composed' is an anagram indicator.
'trial' with letters rearranged gives 'rtail'.
'cu' placed inside 'rtail' is 'CURTAIL'.
'in' is the link.
(Other definitions for curtail that I've seen before include "Cut short (an activity)" , "Shorten, abridge" , "Cut short like canine appendage" , "Dock" , "'Cut short, restrict (7)'" .)