Rather alert, possibly grabbing gold eventually (6,2,5)
I believe the answer is:
sooner or later
'eventually' is the definition.
('sooner or later' can be a synonym of 'eventually')
'rather alert possibly grabbing gold' is the wordplay.
'rather' becomes 'sooner' (I've seen this in other clues).
'possibly' indicates anagramming the letters.
'grabbing' indicates putting letters inside.
'gold' becomes 'or' ('or' is the heraldic name for gold).
'alert' with letters rearranged gives 'later'.
'later' placed around 'or' is 'orlater'.
'sooner'+'orlater'='SOONER OR LATER'
(Other definitions for sooner or later that I've seen before include "offering alternative times" , "Errors (a lot) one corrected, eventually" , "not on the dot" , "Inevitably" , "At some point" .)