Count up Es in street — most distressing! (7)
I believe the answer is:
'most distressing' is the definition.
(sad can mean distressing or upsetting)
'count up es in street' is the wordplay.
'count up' becomes 'add' (I've seen this in another clue).
'in' means one lot of letters goes inside another.
'street' becomes 'st' (abbreviation).
'addes' going inside 'st' is 'SADDEST'.
(Other definitions for saddest that I've seen before include "Most blue" , "Unhappiest" , "Most gloomy" , "Most unhappy" , "Most doleful" .)