Snoop quietly by railway (3)
I believe the answer is:
pry
'snoop' is the definition.
(I've seen this before)
'quietly by railway' is the wordplay.
'quietly' becomes 'p' (p for piano and quiet).
'by' says to put letters next to each other.
'railway' becomes 'ry' (abbreviation).
'p'+'ry'='PRY'
(Other definitions for pry that I've seen before include "Be nosey" , "Intrusively enquire" , "Look intrusively" , "Inquire intrusively" , "Interfere" .)