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' is an insertion indicator (to cross can mean to go over or around).
'railway' becomes 'ry' (abbreviation).
'line' becomes 'l' (used when specifying particular lines from a poem).
'ry'+'l'='ryl'
'spy' going around 'ryl' is 'SPRYLY'.
(Other definitions for spryly that I've seen before include "In a nimble or agile manner" , "Actively" .)