Describe former plot to capture island (7)
I believe the answer is:
explain
'describe' is the definition.
(I've seen this before)
'former plot to capture island' is the wordplay.
'former' becomes 'ex' (prefix meaning former or onetime).
'plot' becomes 'plan' (plot can mean to conspire or plan).
'to capture' is an insertion indicator.
'island' becomes 'i' (geographical abbreviation).
'plan' placed around 'i' is 'plain'.
'ex'+'plain'='EXPLAIN'
(Other definitions for explain that I've seen before include "Shed light" , "Make (something) clear" , "Expound" , "Resolve" , "Make understandable" .)