Plan line, the best possible (5)
I believe the answer is:
ideal
'best possible' is the definition.
(I've seen this before)
'plan line' is the wordplay.
'plan' becomes 'idea' (both can mean an intention).
'line' becomes 'l' (used when specifying particular lines from a poem).
'idea'+'l'='IDEAL'
'the' is the link.
(Other definitions for ideal that I've seen before include "Most suitable" , "Perfect for the purpose" , "Conceptual" , "Exemplary" , "Just right" .)