Important not to start a large number (6)
I believe the answer is:
eighty
'a large number' is the definition.
(eighty is a number)
'important not to start' is the wordplay.
'important' becomes 'weighty' (weighty can mean influential or important).
'not to start' means to remove the first letter.
'weighty' with its initial letter taken off is 'EIGHTY'.
(Other definitions for eighty that I've seen before include "'Nineteen -- Four' (novel)" , "22 [FOUR] scores" , "Number" , "XXC" , "Figure" .)