Charlie on island -- it's very hard (8)
I believe the answer is:
concrete
'very hard' is the definition.
'concrete' can be an answer for 'hard' (synonyms). I am unsure of the 'very' bit.
'charlie on island' is the wordplay.
'charlie' becomes 'C' (phonetic alphabet).
'island' becomes 'crete' (Greek island).
'c'+'on'+'crete'='CONCRETE'
'it's' is the link.
Can you help me to learn more?
(Other definitions for concrete that I've seen before include "Strong, hard, building material" , "Actual - stuff made with cement" , "Solid like building material" , "It's composed of sand, cement and water" , "Solid, real" .)