Improvise brief defence to secure chess file (2-3)
I believe the answer is:
ad-lib
'improvise' is the definition.
(I've seen this before)
'brief defence to secure chess file' is the wordplay.
'brief' means to remove the last letter (a shorter or briefer version of the word).
'defence' becomes 'alibi' (defence given to police by a suspect).
'to secure' means one lot of letters goes inside another (inserted letters are secured inside).
'chess file' becomes 'd' (I can't justify this - if you can you should believe this answer much more).
'alibi' with its final letter removed is 'alib'.
'alib' placed around 'd' is 'AD-LIB'.
Can you help me to learn more?
(Other definitions for ad-lib that I've seen before include "Unprepared remark" , "Improvise" , "Spontaneously" , "Speak off the cuff" , "freely" .)