Delay generated by rodent climbing on railway (5)
I believe the answer is:
tarry
'delay' is the definition.
(to tarry is to delay leaving a place)
'rodent climbing on railway' is the wordplay.
'rodent' becomes 'rat' (rat is a kind of rodent).
'climbing' is a reversal indicator (in a down clue, letters climb up).
'on' means one lot of letters go next to another.
'railway' becomes 'ry' (abbreviation for railway).
'rat' backwards is 'tar'.
'tar'+'ry'='TARRY'
'generated by' is the link.
(Other definitions for tarry that I've seen before include "Drag one's feet" , "Like fresh road surface" , "covered with dark liquid" , "perhaps not to split" , "Delay leaving" .)