Letter only composed to inspire another (7)
I believe the answer is:
ypsilon
'another' is the definition.
I know nothing about this answer so I can't judge whether it can be defined by this definition.
'letter only composed to inspire' is the wordplay.
'letter' becomes 'psi' (psi is a kind of letter).
'composed' is an anagram indicator.
'to inspire' indicates putting letters inside (inspire can mean to breathe in).
'only' with letters rearranged gives 'ylon'.
'psi' inserted into 'ylon' is 'YPSILON'.
Can you help me to learn more?
(Another definition for ypsilon that I've seen is " 20th Greek letter".)