Effective kitty may hold up after taxes (6)
I believe the answer is:
potent
'effective' is the definition.
(associated in meaning)
'kitty may hold up after taxes' is the wordplay.
'kitty may' becomes 'pot' (I can't explain this - if you can you should believe this answer much more).
'hold' is an insertion indicator.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'after taxes' becomes 'net' (as in net profits).
'net' in reverse letter order is 'ten'.
'pot' placed around 'ten' is 'POTENT'.
Can you help me to learn more?
(Other definitions for potent that I've seen before include "Strong in operation" , "Forceful" , "Top Ten (anag.)" , "Powerful like strong drink" , "Having great power" .)