The previous one, in a nice plastic cover, was stiff (9)
I believe the answer is:
inelastic
'stiff' is the definition.
(I've seen this before)
'the previous one in a nice plastic cover' is the wordplay.
'the previous' becomes 'last' (I am not sure about this - if you are sure you should believe this answer much more).
'one' becomes 'i' (Roman numeral).
'in' means one lot of letters goes inside another.
'plastic' is an anagram indicator (plastic can mean easily moulded).
'cover' means one lot of letters goes inside another.
'nice' anagrammed gives 'inec'.
'i' put within 'inec' is 'ineic'.
'last' placed inside 'ineic' is 'INELASTIC'.
'was' is the link.
Can you help me to learn more?
(Other definitions for inelastic that I've seen before include "Unyielding" , "unable to bend?" , "Not stretchy" , "Not giving" , "Having no give" .)