Into the bargain basement go who, last of all? (3)
I believe the answer is:
too
'into the bargain' is the definition.
(thesaurus)
'basement go who last of all?' is the wordplay.
'last of all?' says to take the final letters (I've seen 'last of all' mean this).
The final letters of 'basement go who' are 'TOO'.
(Other definitions for too that I've seen before include "What a pity!" , "Unduly" , "Furthermore" , "To a greater than desirable extent" , "Also; overly" .)