Be evasive about taking in island horse (8)
I believe the answer is:
Here is my best explanation:
'horse' is the definition.
(I've seen this before)
'be evasive about taking in island' is the subsidiary indication.
'be evasive' becomes 'stall' (I've seen this before).
'about' becomes 'on' (**).
'taking in' indicates putting letters inside.
'island' becomes 'i'.
'stallon' placed around 'i' is 'stallion'.
(Other definitions for stallion that I've seen before include "Stud", "Male horse kept for breeding", "Horse kept for breeding", "Steed", "stud", "Male horse, often used for breeding", "Mare's mate", "Male horse lost in atolls", "Mount" and "Uncastrated adult male horse".)