The character that put the “To Rent” sign outside? (6)
I believe the answer is:
letter
'the character' is the definition.
(letter is a kind of character)
'put the to rent sign outside?' is the wordplay.
'put' indicates putting letters inside.
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'to rent' becomes 'let' ('let' can be a synonym of 'rent').
'sign outside?' becomes 'er' (I can't justify this - if you can you should give a lot more credence to this answer).
't' going into 'let' is 'lett'.
'lett'+'er'='LETTER'
'that' acts as a link.
Can you help me to learn more?
(Other definitions for letter that I've seen before include "Character of the alphabet" , "Eg, A" , "Unit of alphabet" , "It's one of twenty-six" , "Character - landlord - epistle" .)