As a rule, information gathering consumes energy (9)
I believe the answer is:
generally
'as a rule' is the definition.
('as a rule' means generally or typically)
'information gathering consumes energy' is the wordplay.
'information' becomes 'gen' (gen can informally mean information).
'gathering' becomes 'rally' (rally is a kind of gathering).
'consumes' means one lot of letters goes inside another.
'energy' becomes 'e' (physics symbol as in E=mca).
'gen'+'rally'='genrally'
'genrally' enclosing 'e' is 'GENERALLY'.
(Other definitions for generally that I've seen before include "In most cases, as a general rule" , "For the most part" , "as usual" , "most of the time" , "Usually, as a rule" .)