Without risk, like a strongbox — extremely lucky! (6)
I believe the answer is:
safely
'without risk' is the definition.
(I've seen this in another clue)
'strongbox extremely lucky' is the wordplay.
'strongbox' becomes 'safe' (safe is a kind of strongbox).
'extremely' means to remove the middle letters (only the extremes of the word are used).
'lucky' with its centre removed is 'ly'.
'safe'+'ly'='SAFELY'
'like a' is the link.
(Other definitions for safely that I've seen before include "In a manner not exposed to danger" , "In a secure manner" , "Without danger" , "In a risk-free manner" .)