Old axes then again taken into battle, ridiculous things (9)
I believe the answer is:
oxymorons
'ridiculous things' is the definition.
'old axes then again taken into battle' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'axes' becomes 'xy' (x axis and y axis).
'then again' becomes 'or' (both can mean 'alternatively').
'taken into' means one lot of letters goes inside another.
'battle' becomes 'mons' (I've seen this before).
'or' going into 'mons' is 'morons'.
'o'+'xy'+'morons'='OXYMORONS'
(Other definitions for oxymorons that I've seen before include "apparent contradictions" , "Contradictions in terms" , "Figures of speech with an apparent contradiction" .)