Line nailed by author, only just (6)
I believe the answer is:
hardly
'only just' is the definition.
(I've seen this before)
'line nailed by author' is the wordplay.
'line' becomes 'l' (used when specifying particular lines from a poem).
'nailed by' is an insertion indicator.
'author' becomes 'hardy' (Thomas Hardy).
'l' going inside 'hardy' is 'HARDLY'.
(Other definitions for hardly that I've seen before include "Almost not" , "Not much" , "Not quite "It seems unlikely!"" , "With great difficulty" , "Barely, scarcely" .)