A number happening to be in the fringes of society (7)
I believe the answer is:
seventy
'a number' is the definition.
(seventy is a kind of number)
'happening to be in the fringes of society' is the wordplay.
'happening' becomes 'event' (I've seen this before**).
'to be in the' means one lot of letters goes inside another.
'fringes of' says to hollow out the word (remove centre letters).
'society' with its middle removed is 'sy'.
'event' placed within 'sy' is 'SEVENTY'.
(Other definitions for seventy that I've seen before include "Threescore and ten" , "LXX" , "Three score years and ten" , "A number" .)