Fourteen letters on a line offering no key (6)
I believe the answer is:
atonal
'fourteen' is the definition.
Both the definition and answer are adjectives. Perhaps you can see an association between them that I can't see?
'letters on a line offering no key' is the wordplay.
'letters on' becomes 'aton' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'line' becomes 'el' (el is a kind of line).
'offering no' suggests deleting specific letters.
'key' becomes 'e' (musical key).
'el' with 'e' taken away is 'l'.
'aton'+'a'+'l'='ATONAL'
Can you help me to learn more?
(Other definitions for atonal that I've seen before include "(Of music) not in any key or mode" , "Musically, having no established key" , "Describes music not written in a particular key" , "Not in a key (mus.)" , "(Music) having no established key" .)