Speculation those people kept about soldiers (6)
I believe the answer is:
theory
'speculation' is the definition.
(something theorised or speculated)
'those people kept about soldiers' is the wordplay.
'those people' becomes 'they' (they refers to a group of people).
'kept' means one lot of letters goes inside another.
'about soldiers' becomes 'or' (Other Ranks. I am not sure about the 'about' bit.).
'they' enclosing 'or' is 'THEORY'.
Can you help me to learn more?
(Other definitions for theory that I've seen before include "Scientific conjecture" , "abstract reasoning" , "Supposition or idea" , "Supposition intended to explain observations" , "Ideal situation" .)