“Down” different initially, while going up (3)
I believe the answer is:
sad
'down' is the definition.
(I've seen this in another clue)
'different initially while going up' is the wordplay.
'initially' indicates taking the first letters.
'while' becomes 'as' (eg 'while I walked' means 'as I walked').
'going up' is a reversal indicator (in a down clue, the letters are written upwards).
The initial letter of 'different' is 'd'.
'd'+'as'='das'
'das' reversed gives 'SAD'.
(Other definitions for sad that I've seen before include "Disconsolate" , "Dejected" , "Despondent" , "Dismal" , "Melancholy, depressed" .)