Characters help a bat in trouble (8)
I believe the answer is:
alphabet
'characters' is the definition.
(I've seen this before)
'help a bat in trouble' is the wordplay.
'in trouble' indicates an anagram.
'help'+'a'+'bat'='helpabat'
'helpabat' anagrammed gives 'ALPHABET'.
(Other definitions for alphabet that I've seen before include "Group of characters" , "First elements (of language)" , "series of letters" , "Depend (on)" , "filling in crosswords" .)