Work needed on Post Office location over the road (8)
I believe the answer is:
opposite
'over the road' is the definition.
'work needed on post office location' is the wordplay.
'work' becomes 'op' (common abbreviation for 'opus').
'needed on' says to put letters next to each other (in a down clue, some letters go on top of others).
'post office' becomes 'po'.
'location' becomes 'site' (site is a kind of location).
'op'+'po'+'site'='OPPOSITE'
(Other definitions for opposite that I've seen before include "Directly facing each other" , "Oppose it, being faced against" , "Contrary, antipode" , "Corresponding" , "reverse, in fact" .)