A soldier at Waterloo, say, heading off trouble (9)
I believe the answer is:
agitation
'trouble' is the definition.
(thesaurus)
'a soldier at waterloo say heading off' is the wordplay.
'soldier' becomes 'gi' (government issue or general issue).
'at' says to put letters next to each other.
'waterloo say' becomes 'station' (Waterloo is a London Underground station).
'heading off' means to remove the first letter.
'station' with its first letter removed is 'tation'.
'a'+'gi'+'tation'='AGITATION'
(Other definitions for agitation that I've seen before include "Nervousness" , "Disturbance -- upheaval" , "The stirring up of trouble" , "disquiet" , "Stir" .)