Film about mutant salt, a threat to sea life? (7)
I believe the answer is:
plastic
'a threat to sea life?' is the definition.
(plastic is dangerous to sea life)
'film about mutant salt' is the wordplay.
'film' becomes 'pic' (short for picture).
'about' means one lot of letters goes inside another.
'mutant' indicates anagramming the letters.
'salt' is an anagram of 'last'.
'pic' enclosing 'last' is 'PLASTIC'.
(Other definitions for plastic that I've seen before include "Generic name for certain synthetic materials" , "means of deferring payment" , "Ubiquitous synthetic material" , "Artificial; easily moulded" , "Credit cards etc" .)