Get at a can on the outside (6)
I believe the answer is:
attain
'get' is the definition.
(attaining is a kind of getting)
'at a can on the outside' is the wordplay.
'can' becomes 'tin' (both are metal containers).
'on the outside' is an insertion indicator.
'a' going inside 'tin' is 'tain'.
'at'+'tain'='ATTAIN'
(Other definitions for attain that I've seen before include "Reach (a specified level)" , "Manage to get" , "Reach (a specified age)" , "Reach (age, size etc.)" , "Succeed in reaching, achieve" .)