Careful, however, holding gold (8)
I believe the answer is:
thorough
'careful' is the definition.
('thorough' can be similar in meaning to 'careful')
'however holding gold' is the wordplay.
'however' becomes 'though' ('though' can be a synonym of 'however').
'holding' indicates putting letters inside.
'gold' becomes 'or' ('or' is the heraldic name for gold).
'though' enclosing 'or' is 'THOROUGH'.
(Other definitions for thorough that I've seen before include "Full" , "Complete and careful" , "Miticulous or exhaustive" , "Scrupulous in completing work" , "Total - painstaking - in-depth" .)