Mole crossing railway line with agility (6)
I believe the answer is:
spryly
'with agility' is the definition.
(I've seen this in another clue)
'mole crossing railway line' is the wordplay.
'mole' becomes 'spy' (I've seen this before).
'crossing' means one lot of letters goes inside another (to cross can mean to go over or around).
'railway' becomes 'ry' (abbreviation for railway).
'line' becomes 'l' (used when specifying particular lines from a poem).
'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" .)