Speculating in article on old rebellion (10)
I believe the answer is:
theorising
'speculating' is the definition.
('theorise' can be a synonym of 'speculate')
'article on old rebellion' is the wordplay.
'article' becomes 'the' (the 'definite article' part of speech).
'on' means one lot of letters go next to another.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'rebellion' becomes 'rising' ('rising' can be a synonym of 'rebellion').
'the'+'o'+'rising'='THEORISING'
'in' is the link.
(Other definitions for theorising that I've seen before include "In speculative activity" , "indulging in speculation" , "coming up with ideas?" .)