Chemical cover including aluminium (6)
I believe the answer is:
halide
'chemical' is the definition.
(thesaurus)
'cover including aluminium' is the wordplay.
'cover' becomes 'hide' (hide is a kind of cover).
'including' indicates putting letters inside.
'aluminium' becomes 'al' (Al is the chemical symbol for aluminium).
'hide' going around 'al' is 'HALIDE'.
(Other definitions for halide that I've seen before include "Halogen compound" , "Chemical containing e g chlorine" .)