Listed site, heading off, is in sea (8)
I believe the answer is:
itemised
'listed' is the definition.
(I've seen this before)
'site heading off is in sea' is the wordplay.
'heading off' means to remove the first letter.
'in' indicates putting letters inside.
'sea' becomes 'med' (abbreviation of Mediterranean).
'site' with its first letter taken away is 'ite'.
'is' going into 'med' is 'mised'.
'ite'+'mised'='ITEMISED'
(Other definitions for itemised that I've seen before include "placed on list" , "Ticked off one by one" , "Listed individually" , "Specified in a list" , "Given separately in a list" .)