Is the number of years for one! (4)
I believe the answer is:
itch
'is the number of years for one' is the definition.
I can't tell whether this defines the answer.
'the number of years for one' is the wordplay.
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'number of years' becomes 'ch' (I can't justify this - if you can you should believe this answer much more).
'for' is a charade indicator (letters next to each other) (I've seen this in other clues).
'one' becomes 'i' (Roman numeral).
't'+'ch'='tch'
'tch' put after 'i' is 'ITCH'.
Can you help me to learn more?
(Other definitions for itch that I've seen before include "Impatient desire (colloq.)" , "Hunger, longing" , "Restless longing" , "are dying" , "Burning" .)