Silly person crosses one line in the half-dark (6)
I believe the answer is:
twilit
'dark' is the definition.
(similar in meaning)
'silly person crosses one line in the half' is the wordplay.
'silly person' becomes 'twit' (I've seen this before).
'crosses' indicates putting letters inside.
'one line' becomes 'l' (used when specifying particular lines from a poem).
'the half' means to take half the letters.
'in' with half the letters taken is 'i'.
'l'+'i'='li'
'twit' enclosing 'li' is 'TWILIT'.
(Other definitions for twilit that I've seen before include "Somewhat dim" , "dusky?" , "Road surfacing" , "Dimly illuminated" , "Gently illuminated" .)