Gravity, for instance (5)
I believe the answer is:
force
'gravity for instance' is the definition.
(I know that gravitation is a type of attraction)
This is all the clue.
(Other definitions for force that I've seen before include "Power; army" , "Impel, coerce" , "Act of God preventing someone from fulfilling a contract" , "Coercion; influence" , "Energy" .)