Encountered crook regularly in underground (5)
I believe the answer is:
metro
'underground' is the definition.
(I know that underground can be written as metro)
'encountered crook regularly' is the wordplay.
'encountered' becomes 'met' ('meet' can be a synonym of 'encounter').
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
The alternate letters of 'crook' are 'ro'.
'met'+'ro'='METRO'
'in' is the link.
(Other definitions for metro that I've seen before include "Parisian underground railway" , "Parisian underground system" , "The underground railway system in Paris" , "Paris subway" , "The Paris Underground transport system" .)