Opening letter, I nail it up the wall (7)
I believe the answer is:
initial
'wall' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
'opening letter i nail it up' is the wordplay.
'opening' indicates putting letters inside (some letters open a gap in others).
'letter' is an anagram indicator (I've seen 'letters' mean this).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'it' reversed gives 'ti'.
'i'+'nail'='inail'
'inail' anagrammed gives 'inial'.
'inial' enclosing 'ti' is 'INITIAL'.
'the' acts as a link.
Can you help me to learn more?
(Other definitions for initial that I've seen before include "Original; beginning" , "Letter - opening" , "First letter of a name" , "E in Eileen" , "Occurring at the beginning" .)