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' is an anagram indicator.
'about' means one lot of letters goes inside another.
'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" .)