Number left after the start of the year (6)
I believe the answer is:
twenty
'number' is the definition.
(twenty is a kind of number)
'left after the start of the year' is the wordplay.
'left' becomes 'went' (I've seen this before**).
'after' is a charade indicator (letters next to each other).
'the start of' says to take the initial letters.
'year' becomes 'y'.
The first letter of 'the' is 't'.
'went' put after 't' is 'twent'.
'twent'+'y'='TWENTY'
(Other definitions for twenty that I've seen before include "Hundredweights to a ton" , "John B Keane wrote about many young men of this age" , "One score" , "No longer a teenager" , "Cardinal" .)