Something added to letter gets animated backing (5)
I believe the answer is:
serif
'something added to letter' is the definition.
(feature of a letter in a typeface)
'gets animated backing' is the wordplay.
'gets animated' becomes 'fires'.
'backing' is a reversal indicator.
'fires' back-to-front is 'SERIF'.
(Other definitions for serif that I've seen before include "Typeface decoration" , "addition to character?" , "Part of letter" , "One feature of text" , "Slight projection finishing off printed letter - puts fires out" .)