Being shifty, I have to save up beforehand (7)
I believe the answer is:
evasive
'being shifty' is the definition.
(I've seen this in another clue)
'i have to save up beforehand' is the wordplay.
'i have to' becomes 'ive' (I can't explain this - if you can you should give a lot more credence to this answer).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'beforehand' is a charade indicator (letters next to each other).
'save' reversed gives 'evas'.
'ive' put after 'evas' is 'EVASIVE'.
Can you help me to learn more?
(Other definitions for evasive that I've seen before include "Difficult to pin down" , "Devious" , "Prevaricating" , "Hard to catch" , "Shifty; equivocating" .)