Drive back social outcast that must be put up (5)
I believe the answer is:
repel
'drive back social outcast that' is the definition.
'repel' can be an answer for 'drive back' (thesaurus). I am not sure about the rest of the definition.
'must be put up' is the wordplay.
'must' becomes 'le' (I can't explain this - if you can you should believe this answer much more).
'be put' becomes 'per'.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'le'+'per'='leper'
'leper' back-to-front is 'REPEL'.
Can you help me to learn more?
(Other definitions for repel that I've seen before include "push off" , "Be effective in controlling" , "oppose" , "Be impervious to" , "Drive back, rebuff" .)