It's been built for a bit of a battle, just in case (6)
I believe the answer is:
castle
'it's been built for a bit' is the definition.
I can't tell whether this definition defines the answer.
'a battle just in case' is the wordplay.
'a battle just' becomes 'tl' (I am not sure about this - if you are sure you should believe this answer much more).
'in' indicates putting letters inside.
'tl' inserted within 'case' is 'CASTLE'.
'of' acts as a link.
Can you help me to learn more?
(Other definitions for castle that I've seen before include "Let sac be a chess piece" , "Lincoln, Ludlow or Leeds" , "Dover, perhaps" , "making a move" , "the home of a Sassenach?" .)