2 in 100 Poles have a duty to go inside (7)
I believe the answer is:
noughts
'2 in 100' is the definition.
(there are two zeroes in 100)
'poles have a duty to go inside' is the wordplay.
'poles' becomes 'ns' (North and South poles).
'have a duty' becomes 'ought' (if you ought to do something it is your duty).
'to go inside' means one lot of letters goes inside another.
'ns' going around 'ought' is 'NOUGHTS'.
(Other definitions for noughts that I've seen before include "Game played with pencil and paper" , "Zeroes" , "- - and crosses" , "Zeros" , "More than once, zero" .)