Substantial clubs on Greek island (8)
I believe the answer is:
concrete
'substantial' is the definition.
(thesaurus)
'clubs on greek island' is the wordplay.
'clubs' becomes 'c' (abbreviation used in card games such as bridge).
'greek island' becomes 'crete' (Crete is a Greek island).
'c'+'on'+'crete'='CONCRETE'
(Other definitions for concrete that I've seen before include "Not abstract or imaginary" , "Substantial, real" , "Firm" , "It's composed of sand, cement and water" , "setter" .)