Host’s mooring on island (9)
I believe the answer is:
anchorman
'host's' is the definition.
(I've seen this before)
'mooring on island' is the wordplay.
'mooring' becomes 'anchor' (synonyms).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'island' becomes 'Man' (the Isle of Man is an example).
'anchor'+'man'='ANCHORMAN'
(Other definitions for anchorman that I've seen before include "host" , "Crucial participant" , "Last person in relay race - TV presenter" , "with weight on board?" , "Last runner in a relay race - person presenting a TV show" .)