Item in trunk you are missing in British seaside resort (10)
I believe the answer is:
breastbone
'item in trunk' is the definition.
(part of a person's torso or trunk)
'you are missing in british seaside resort' is the wordplay.
'you are' becomes 'ur' (text or internet abbreviation).
'missing in' indicates named letters should be taken away.
'british' becomes 'br'.
'seaside resort' becomes 'Eastbourne' (English resort town).
'br'+'eastbourne'='breastbourne'
'breastbourne' with 'ur' taken out is 'BREASTBONE'.
(Other definitions for breastbone that I've seen before include "Rib neighbour" , "Sternum" .)