Copper on trial, composed in dock (7)
I believe the answer is:
curtail
'dock' is the definition.
(I've seen this before)
'copper on trial composed' is the wordplay.
'copper' becomes 'cu' (chemical symbol**).
'on' means one lot of letters goes inside another.
'composed' indicates an anagram.
'trial' with letters rearranged gives 'rtail'.
'cu' put into 'rtail' is 'CURTAIL'.
'in' is the link.
(Other definitions for curtail that I've seen before include "Reduce in length" , "Dock" , "Cut short (an activity)" , "'Cut short, restrict (7)'" , "Cut short like canine appendage" .)