Undermine engagement taking pressure for stone (8)
I believe the answer is:
sapphire
'stone' is the definition.
(I've seen this before)
'undermine engagement taking pressure' is the wordplay.
'undermine' becomes 'sap' (I've seen this before).
'engagement' becomes 'hire' ('hire' can be a synonym of 'engagement').
'taking' is an insertion indicator.
'pressure' becomes 'p' (abbreviation).
'sap'+'hire'='saphire'
'saphire' placed around 'p' is 'SAPPHIRE'.
'for' is the link.
(Other definitions for sapphire that I've seen before include "Deep blue colour" , "His paper can be turned into a jewel" , "Birthstone for September" , "Corundum gemstone" , "His paper can be made into a jewel" .)