Element of doubt expressed in a fool having first made a comeback (8)
I believe the answer is:
titanium
'element' is the definition.
(titanium is a kind of element)
'doubt expressed in a fool having first made a comeback' is the wordplay.
I cannot really see how this works, but
'a' could be 'an' and 'an' is located in the answer.
'fool' could be 'tit' (I've seen this in other clues) and 'tit' is found within the answer.
'first' could be 'i' (I in Roman numerals eg Elizabeth I) and 'i' is present in the answer.
The remaining letters 'um' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'of' acts as a link.
Can you help me to learn more?
(Other definitions for titanium that I've seen before include "Metal used in strong lightweight alloys" , "Element 22, makes strong non-corroding alloys" , "Light strong metallic element used in aeroplane parts" , "Atomic no. 22" , "Metal used in the manufacture of light alloys for aircraft parts" .)