Security guard reduced stake by a quarter (9)
I believe the answer is:
guarantee
'security' is the definition.
(guarantee is a kind of security)
'guard reduced stake by a quarter' is the wordplay.
'reduced' means to remove the last letter.
'stake' becomes 'ante' (stake in poker).
'by' says to put letters next to each other.
'a quarter' becomes 'e'.
'guard' with its final letter removed is 'guar'.
'guar'+'ante'+'e'='GUARANTEE'
(Other definitions for guarantee that I've seen before include "Written assurance, pledge" , "Something ensuring a particular outcome" , "Promising" , "Undertake" , "Ensure" .)