Defy youth at regular intervals in small island (4)
I believe the answer is:
eyot
'small island' is the definition.
(I've seen this before)
'defy youth at regular intervals' is the wordplay.
'at regular intervals' means one should take alternating letters.
'defy'+'youth'='defyyouth'
The alternate letters of 'defyyouth' are 'EYOT'.
'in' is the link.
(Other definitions for eyot that I've seen before include "isle" , "Islet found in a river, (especially in England)" , "Another term for ait" , "land in the river" , "Small river island" .)