Joins me on the last back street (5)
I believe the answer is:
meets
'joins' is the definition.
(meeting is a kind of joining)
'me on the last back street' is the wordplay.
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'last' says to take the final letters.
'back' is a reversal indicator.
'street' becomes 'st' (abbreviation).
The last letter of 'the' is 'e'.
'st' in reverse letter order is 'ts'.
'me'+'e'+'ts'='MEETS'
(Other definitions for meets that I've seen before include "Gathers together" , "Comes into contact with" , "Makes contact with" , "Hunts" , "Effectively copes with" .)