Island where unknown character has broken bone, having struck head (5)
I believe the answer is:
ibiza
'island' is the definition.
(Spanish island)
'unknown character has broken bone having struck head' is the wordplay.
'unknown character' becomes 'z' (algebra).
'has broken' is an insertion indicator (inserted letters break in).
'bone' becomes 'tibia' (tibia is a kind of bone).
'having struck head' means to remove the first letter.
'tibia' with its initial letter taken away is 'ibia'.
'z' going within 'ibia' is 'IBIZA'.
'where' acts as a link.
(Other definitions for ibiza that I've seen before include "Mediterranean island" , "One of the Balearic Islands" , "Spanish island" , "Westermost of Balearic Islands, popular for holidays" , "Island in the Balearics" .)