Letter, a ticklish thing (5)
I believe the answer is:
aitch
'letter' is the definition.
(aitch is a kind of letter)
'a ticklish thing' is the wordplay.
'ticklish thing' becomes 'itch' (I can't explain this - if you can you should give a lot more credence to this answer).
'a'+'itch'='AITCH'
Can you help me to learn more?
(Other definitions for aitch that I've seen before include "that I will immediately succeed" , "character leading heroes" , "It's dropped by Cockneys" , "Eighth letter of the English alphabet" , "8th letter of the alphabet" .)