Second English lesson is potentially distinct (9)
I believe the answer is:
separable
'potentially distinct' is the definition.
(able to be separated or made distinct)
'second english lesson' is the wordplay.
'second' becomes 's' (common abbreviation).
'english' becomes 'e' (abbreviation).
'lesson' becomes 'parable' (I've seen this before).
's'+'e'+'parable'='SEPARABLE'
'is' acts as a link.
(Other definitions for separable that I've seen before include "not permanently joined together?" , "That may be disjoined" .)