Essential island's maintained by Arctic supply line (8)
I believe the answer is:
critical
'essential' is the definition.
(I've seen this before)
'island's maintained by arctic supply line' is the wordplay.
'island' becomes 'i' (geographical abbreviation).
'maintained by' is an insertion indicator (maintain can mean to keep hold of).
'supply' is an anagram indicator (in a supple way).
'line' becomes 'l' (used when specifying particular lines from a poem).
'arctic' anagrammed gives 'crtica'.
'i' put into 'crtica' is 'critica'.
'critica'+'l'='CRITICAL'
(Other definitions for critical that I've seen before include "Decisively important" , "Making negative judgments" , "Decisive - grave" , "expressing disapproving comments" , "Seriously ill" .)