Perfect notion to start learning
I believe the answer is:
ideal
'perfect' is the definition.
(similar in meaning)
'notion to start learning' is the wordplay.
'notion' becomes 'idea' (I've seen this in another clue).
'to start' suggests taking the first letters.
The first letter of 'learning' is 'l'.
'idea'+'l'='IDEAL'
(Other definitions for ideal that I've seen before include "The best conceivable" , "without flaws" , "Standard to aim for" , "Just right" , "Conceptual" .)