Shortened train addressed complaint of guards (9)
I believe the answer is:
curtailed
'shortened' is the definition.
(curtailing is a kind of shortening)
'train addressed complaint of guards' is the wordplay.
'train' becomes 'tail' (I've seen this before).
'addressed complaint of' becomes 'cured' (I can't explain this - if you can you should believe this answer much more).
'guards' indicates putting letters inside (inserted letters are guarded).
'tail' placed into 'cured' is 'CURTAILED'.
Can you help me to learn more?
(Other definitions for curtailed that I've seen before include "limited" , "Broken off" , "Cut short" , "docked" , "Clipped" .)