Animated Roman historian pens article in Spanish (6)
I believe the answer is:
lively
'animated' is the definition.
(similar in meaning)
'roman historian pens article in spanish' is the wordplay.
'roman historian' becomes 'livy' (I've seen this before).
'pens' indicates putting letters inside (inserted letters are held or penned in).
'article in spanish' becomes 'el' ('el' is a definite article in Spanish).
'livy' going around 'el' is 'LIVELY'.
(Other definitions for lively that I've seen before include "Con brio" , "Full of fun" , "Full of zest and energy" , "Vivacious" , "Full of zest or vigour" .)