Doctrine distributing material gains across island (14)
I believe the answer is:
egalitarianism
'doctrine' is the definition.
(egalitarianism is a kind of doctrine)
'distributing material gains across island' is the wordplay.
'distributing' is an anagram indicator.
'across' means one lot of letters goes inside another.
'island' becomes 'i'.
'material'+'gains'='materialgains'
'materialgains' with letters rearranged gives 'egalitariansm'.
'egalitariansm' going around 'i' is 'EGALITARIANISM'.
(Other definitions for egalitarianism that I've seen before include "Belief that all are equal" , "radical policy" , "Belief in the equality of men" .)