Trace the first nine letters, backwards (4)
I believe the answer is:
iota
'trace' is the definition.
(I've seen this before)
'first nine letters backwards' is the wordplay.
'first' becomes 'a'.
'nine letters' becomes 'toi' (I can't explain this - if you can you should give a lot more credence to this answer).
'backwards' shows that the letters should be reversed in order.
'a'+'toi'='atoi'
'atoi' in reverse letter order is 'IOTA'.
'the' is the link.
Can you help me to learn more?
(Other definitions for iota that I've seen before include "Gr. letter; small amount" , "A little piece" , "Trace" , "'Very small amount, jot (4)'" , "Tiny bit" .)