Permissive character? (6)

I believe the answer is:

letter

'character?' is the definition.
(synonyms)

I can't explain the remainder of the clue.

(Other definitions for letter that I've seen before include "Unit of alphabet" , "Character; missive" , "who may offer terms to residents?" , "It's one of twenty-six" , "Eg, A" .)

I've seen this clue in The Telegraph, The Times and The Sun.

Want a hint initially instead of a full solution? Install my app