Solid, head of clan on Greek island (8)
I believe the answer is:
concrete
'solid' is the definition.
('concrete' can be similar in meaning to 'solid')
'head of clan on greek island' is the wordplay.
'head of' says to take the initial letters.
'greek island' becomes 'crete' (I've seen this before).
The initial letter of 'clan' is 'c'.
'c'+'on'+'crete'='CONCRETE'
(Other definitions for concrete that I've seen before include "Firm" , "Specific - building material" , "Real, not abstract" , "setter" , "Building material is not abstract" .)