Substance affected Michael Caine's head (8)
I believe the answer is:
chemical
'substance' is the definition.
(chemical is a kind of substance)
'affected michael caine's head' is the wordplay.
'affected' is an anagram indicator.
'head' says to take the initial letters.
The initial letter of 'caines' is 'c'.
'michael'+'c'='michaelc'
'michaelc' is an anagram of 'CHEMICAL'.
(Other definitions for chemical that I've seen before include "Material produced by a reaction involving changes in atoms or molecules" , "Chic male (anag.)" , "Weedkiller or pesticide, say" , "substance produced in a lab" , "Like 4 or 24 dn" .)