Arm reinforcements organising raid to capture island (5)
I believe the answer is:
radii
'arm reinforcements' is the definition.
Although both the answer and definition are plural nouns, I can't see how they can define each other.
'organising raid to capture island' is the wordplay.
'organising' is an anagram indicator.
'to capture' means one lot of letters goes inside another.
'island' becomes 'i'.
'raid' with letters rearranged gives 'radi'.
'radi' enclosing 'i' is 'RADII'.
Can you help me to learn more?
(Other definitions for radii that I've seen before include "Straight lines from centre of a circle" , "Lines between the center and circumference of a circle" , "Straight lines from the centre to the surface of a sphere" , "Lines from a circle's centre to the circumference" , "parts for arms" .)