Cross the Rubicon - initially reluctant (8)
I believe the answer is:
traverse
'cross' is the definition.
(I know that cross can be written as traverse)
'the rubicon initially reluctant' is the wordplay.
'initially' says to take the initial letters.
'reluctant' becomes 'averse' (averse can mean reluctant or opposing).
The first letters of 'the rubicon' is 'tr'.
'tr'+'averse'='TRAVERSE'
(Other definitions for traverse that I've seen before include "Ski diagonally across (a slope)" , "Go across" , "cover" , "Extend or bridge across" , "Move across" .)