Island secures contract with Georgia for chemical quantity (5,3)
I believe the answer is:
ideal gas
'chemical quantity' is the definition.
(hypothetical gas)
'island secures contract with georgia' is the wordplay.
'island' becomes 'is'.
'secures' means one lot of letters goes inside another.
'contract' becomes 'deal' (I've seen this before).
'with' is a charade indicator (letters next to each other).
'georgia' becomes 'ga'.
'deal'+'ga'='dealga'
'is' going around 'dealga' is 'IDEAL GAS'.
'for' is the link.
(Other definitions for ideal gas that I've seen before include "Fluid obeying a theory in chemistry" , "that's chemically perfect?" , "Theoretical perfect substance" , "Perfectly-behaving substance in chemistry" , "Theoretical perfect fluid" .)