It's strange following old rule (7)
I believe the answer is:
foreign
'it's strange' is the definition.
('foreign' can be similar in meaning to 'strange')
'following old rule' is the wordplay.
'following' becomes 'f' (used when citing page numbers etc).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'rule' becomes 'reign' (reign is a kind of rule**).
'f'+'o'+'reign'='FOREIGN'
(Other definitions for foreign that I've seen before include "Not native - maybe of Niger" , "Of or from another country" , "Of other countries" , "Unfamiliar; from abroad" , "Non-native" .)