Copper's arresting deed, beginning to search plant (6)
I believe the answer is:
cactus
'plant' is the definition.
(cactus is a kind of plant)
'copper's arresting deed beginning to search' is the wordplay.
'copper' becomes 'cu' (chemical symbol).
'arresting' is an insertion indicator.
'deed' becomes 'act' (I've seen this before).
'beginning to' indicates taking the first letters.
The first letter of 'search' is 's'.
'cu' placed around 'act' is 'cactu'.
'cactu'+'s'='CACTUS'
(Other definitions for cactus that I've seen before include "Spiky desert plant" , "Drought-resistant plant" , "Prickly sort" , "Eg, prickly pear" , "Spiny desert plant" .)