Thought supreme deity initially mad (8)
I believe the answer is:
presumed
'thought' is the definition.
(I've seen this before)
'supreme deity initially mad' is the wordplay.
'initially' suggests taking the first letters.
'mad' is an anagram indicator.
The first letter of 'deity' is 'd'.
'supreme'+'d'='supremed'
'supremed' is an anagram of 'PRESUMED'.
(Other definitions for presumed that I've seen before include "Supposed" , "Was impertinent" , "thought" , "Took for granted" , "Taken for granted" .)