Post Office holding new item in tropical fruit (8)
I believe the answer is:
pimiento
'tropical fruit' is the definition.
(type of pepper)
'post office holding new item in' is the wordplay.
'post office' becomes 'po' (abbreviation).
'holding' indicates putting letters inside.
'new' indicates anagramming the letters.
'item'+'in'='itemin'
'itemin' with letters rearranged gives 'imient'.
'po' placed around 'imient' is 'PIMIENTO'.
(Other definitions for pimiento that I've seen before include "Kitchen ingredient" , "What's sweet yet spicy" , "Sweet red capsicum" , "Red sweet pepper" , "Allspice" .)