Doctrines revealed in island writing (4)
I believe the answer is:
isms
'doctrines' is the definition.
('ism' can be a synonym of 'doctrine')
'island writing' is the wordplay.
'island' becomes 'is' (abbreviation).
'writing' becomes 'ms' (abbreviation for manuscript).
'is'+'ms'='ISMS'
'revealed in' acts as a link.
(Other definitions for isms that I've seen before include "Informal suffixes for doctrines or practices, confused miss (4)" , "Distinctive doctrines" , "'Beliefs, philosophies (4)'" , "Distinctive theories" , "Informal doctrines - near miss" .)