Train in January to hold stealthy fighter (5)
I believe the answer is:
ninja
'stealthy fighter' is the definition.
(I've seen this in another clue)
'train in january to hold' is the wordplay.
'to hold' says the answer is hidden in the clue (I've seen 'holding' mean this).
'NINJA' is hidden in the letters of 'train in january'.
(Other definitions for ninja that I've seen before include "Stealthy Japanese spy" , "agent from the east" , "Mercenary in feudal Japan" , "One who steals" , "Old Japanese hired assassin" .)