Head of Institute finding nothing about glass substitute unfavourable (8)
I believe the answer is:
inimical
'unfavourable' is the definition.
(I've seen this before)
'head of institute finding nothing about glass substitute' is the wordplay.
'head of' indicates taking the first letters.
'finding' means one lot of letters go next to another.
'nothing' becomes 'nil' (eg nil is zero in football).
'about' is an insertion indicator.
'glass substitute' becomes 'mica' (I've seen this in another clue).
The first letter of 'institute' is 'i'.
'nil' enclosing 'mica' is 'nimical'.
'i'+'nimical'='INIMICAL'
(Other definitions for inimical that I've seen before include "Pernicious" , "Adverse" , "In claim I am strangely hostile" , "Hostile or in opposition" , "Unfriendly or unfavourable" .)