Tally up at the start. No-no (4)
I believe the answer is:
tabu
'no-no' is the definition.
(I've seen this in another clue)
'tally up at the start' is the wordplay.
'tally' becomes 'tab' (I can't explain this - if you can you should give a lot more credence to this answer).
'at the start' suggests taking the first letters.
The initial letter of 'up' is 'u'.
'tab'+'u'='TABU'
Can you help me to learn more?
(Other definitions for tabu that I've seen before include "banned" , "Socially prohibited" , "beyond the pale" , "delicate matter?" , "Ban resulting from social custom" .)