Copper rings keep one supple (7)
I believe the answer is:
plastic
'supple' is the definition.
(thesaurus)
'copper rings keep one' is the wordplay.
'copper' becomes 'PC' (police constable).
'rings' means one lot of letters goes inside another (some letters are ringed or contained by others).
'keep' becomes 'last' (I've seen this in another clue).
'one' becomes 'i' (Roman numeral).
'last'+'i'='lasti'
'pc' going around 'lasti' is 'PLASTIC'.
(Other definitions for plastic that I've seen before include "Synthetic polymer" , "Scalp it badly to make a synthetic material" , "Pliant" , "Clasp it (anag.)" , "(Originally malleable) material" .)