Aren't letters on letterhead a cost of the letter? (6)
I believe the answer is:
rental
'a cost of the letter?' is the definition.
I can't judge whether this defines the answer.
'aren't letters on letterhead' is the wordplay.
'letters' indicates an anagram.
'on' is a charade indicator (letters next to each other).
'letterhead' becomes 'l' (head letter of 'letter').
'arent' with letters rearranged gives 'renta'.
'renta'+'l'='RENTAL'
Can you help me to learn more?
(Other definitions for rental that I've seen before include "Learnt (anag) -- lease" , "Amount paid for hiring something" , "What's offered by a letter" , "Amount paid for hire" , "Let" .)