Bit of a character sacks turns (5)
I believe the answer is:
serif
'bit of a character' is the definition.
(I've seen this before)
'sacks turns' is the wordplay.
'sacks' becomes 'fires' ('fire' can be a synonym of 'sack').
'turns' says the letters should be written backwards.
'fires' back-to-front is 'SERIF'.
(Other definitions for serif that I've seen before include "Slight projection finishing off printed letter - puts fires out" , "Short line at the end of the main stroke of a letter" , "Part of letter" , "Detail on some typefaces" , "decorative line" .)