Encountered rock half underground (5)
I believe the answer is:
metro
'underground' is the definition.
(I know that underground can be written as metro)
'encountered rock half' is the wordplay.
'encountered' becomes 'met' ('meet' can be a synonym of 'encounter').
'half' indicates taking half.
'rock' cut in half is 'ro'.
'met'+'ro'='METRO'
(Other definitions for metro that I've seen before include "Paris underground rail system" , "The Paris Underground transport system" , "Parisian underground system" , "Parisian underground railway" , "The underground railway system in Paris" .)