In short, delivery person that is after work up street (6)
I believe the answer is:
postie
'in short delivery person' is the definition.
(short for postman)
'that is after work up street' is the wordplay.
'that is' becomes 'ie' ('i.e.' can be a synonym of 'that is').
'after' says to put letters next to each other.
'work' becomes 'op' (abbreviation for opus**).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'street' becomes 'st'.
'op' written backwards gives 'po'.
'po'+'st'='post'
'ie' after 'post' is 'POSTIE'.
(Another definition for postie that I've seen is " Mail deliverer (informal)".)