A little problem sending off first letter (5)
I believe the answer is:
aitch
'off first letter' is the definition.
'aitch' can be an answer for 'letter' (I've seen this before). I'm not certain of the 'off first' bit.
'a little problem sending' is the wordplay.
'little problem sending' becomes 'itch' (I am not sure about this - if you are sure you should believe this answer much more).
'a'+'itch'='AITCH'
Can you help me to learn more?
(Other definitions for aitch that I've seen before include "bone" , "8th letter of the alphabet" , "Letter sound that's often dropped?" , "Eighth letter of the English alphabet" , "first bit of hymn?" .)