A number we let off - about five (6)
I believe the answer is:
twelve
'a number' is the definition.
(twelve is a kind of number)
'we let off about five' is the wordplay.
'off' indicates anagramming the letters.
'about' is an insertion indicator.
'five' becomes 'V' (Roman numeral).
'we'+'let'='welet'
'welet' with letters rearranged gives 'twele'.
'twele' enclosing 'v' is 'TWELVE'.
(Other definitions for twelve that I've seen before include "One under a baker's dozen" , "Number of Apostles" , "The midnight hour" , "Half the number here" , "Basis of duodecimal system" .)