Monarchs meet heavyweight in Jamaica (8)
I believe the answer is:
kingston
'in jamaica' is the definition.
('Kingston' is part of 'jamaica')
'monarchs meet heavyweight' is the wordplay.
'monarchs' becomes 'kings' (king is a kind of monarch).
'meet' means one lot of letters go next to another.
'heavyweight' becomes 'ton' (a ton weighs a lot).
'kings'+'ton'='KINGSTON'
(Other definitions for kingston that I've seen before include "Jamaican capital" , "Royal borough on river Thames" , "Capital of Jamaica" , "island capital" .)