A number observed to be about five (5)
I believe the answer is:
seven
'a number' is the definition.
(seven is a kind of number)
'observed to be about five' is the wordplay.
'observed' becomes 'seen' ('seen' can be a synonym of 'observed').
'to be about' means one lot of letters goes inside another.
'five' becomes 'v' (roman numeral).
'seen' going around 'v' is 'SEVEN'.
(Other definitions for seven that I've seen before include "Wonders of the World - Deadly Sins" , "equivalent of uproar [7D]" , "Film" , "number of swans aswimming" , "Cube root of 343" .)