Half-inch nail only at intervals runs all the way to the top (7)
I believe the answer is:
purloin
'all the way to the top' is the definition.
I can't tell whether this defines the answer.
'half-inch nail only at intervals runs' is the wordplay.
'half inch' becomes 'pur' (I can't justify this - if you can you should give a lot more credence to this answer).
'at intervals' means one should take alternating letters (letters are taken at regular intervals).
'runs' indicates anagramming the letters.
'nail'+'only'='nailonly'
The alternate letters of 'nailonly' are 'niol'.
'niol' with letters rearranged gives 'loin'.
'pur'+'loin'='PURLOIN'
Can you help me to learn more?
(Other definitions for purloin that I've seen before include "Steal" , "Make off with" , "'Nick, pilfer (7)'" , "'Nick, filch (7)'" , "appropriate" .)