Very long line I cross, for to get ahead (6)
I believe the answer is:
prolix
'very long' is the definition.
(prolix means very long or wordy)
'line i cross for to get ahead' is the wordplay.
'line' becomes 'l' (used when specifying particular lines from a poem).
'cross' becomes 'x' (X is cross-shaped).
'for' becomes 'pro' (as in being 'pro' something).
'to get ahead' says to put letters next to each other (some letters go ahead of others).
'l'+'i'+'x'='lix'
'lix' put after 'pro' is 'PROLIX'.
(Other definitions for prolix that I've seen before include "Verbose, wordy" , "Tediously wordy and lengthy" , "Wordy, long-winded" , "Using many words" , "Tediously wordy, talks too much" .)