European railway is behind grown-up infidelity (8)
I believe the answer is:
adultery
'infidelity' is the definition.
(I've seen this before)
'european railway is behind grown-up' is the wordplay.
'european' becomes 'E' (abbreviation e.g. EU).
'railway' becomes 'ry' (abbreviation for railway).
'is behind' is a charade indicator (letters next to each other).
'grown-up' becomes 'adult' (informal term for an adult).
'e'+'ry'='ery'
'ery' put after 'adult' is 'ADULTERY'.
(Other definitions for adultery that I've seen before include "Act of infidelity by husband or wife" , "Sexual unfaithfulness to spouse" , "Betrayal of a partner" , "Extramarital bedroom antics" , "Affairs" .)