Agent crossing railway line nimbly (6)
I believe the answer is:
Here is my best explanation:
'nimbly' is the definition.
'agent crossing railway line' is the subsidiary indication.
'agent' becomes 'spy' (spy is a kind of agent).
'crossing' indicates putting letters inside.
'railway' becomes 'ry'.
'line' becomes 'l'.
'spy' going around 'ryl' is 'spryly'.
(Another definition for spryly that I've seen is "In a nimble or agile manner".)