Agent crossing railway line nimbly (6)
I believe the answer is:
spryly
'nimbly' is the definition.
(thesaurus)
'agent crossing railway line' is the wordplay.
'agent' becomes 'spy' (spy is a kind of agent).
'crossing' is an insertion indicator.
'railway' becomes 'ry' (abbreviation for railway).
'line' becomes 'l'.
'ry'+'l'='ryl'
'spy' placed around 'ryl' is 'SPRYLY'.
(Other definitions for spryly that I've seen before include "In a nimble or agile manner" , "Actively" .)