Be evasive about taking in island horse (8)
I believe the answer is:
stallion
'horse' is the definition.
(thesaurus)
'be evasive about taking in island' is the wordplay.
'be evasive' becomes 'stall' (I've seen this before).
'about' becomes 'on' (**).
'taking in' means one lot of letters goes inside another.
'island' becomes 'i'.
'stall'+'on'='stallon'
'stallon' enclosing 'i' is 'STALLION'.
(Other definitions for stallion that I've seen before include "Stud" , "Uncastrated adult male horse" , "Male horse, often used for breeding" , "Mare's mate" , "Male steed" .)