Bones seafarers found on island (5)
I believe the answer is:
tarsi
'bones' is the definition.
(I've seen this before)
'seafarers found on island' is the wordplay.
'seafarers' becomes 'tars' ('tar' can be a synonym of 'seafarer').
'found on' means one lot of letters go next to another.
'island' becomes 'i'.
'tars'+'i'='TARSI'
(Other definitions for tarsi that I've seen before include "Ankle bones" , "parts of leg" , "Bones of the ankles and heels" , "Foot bones" , "collections of bones?" .)