Old man securing a metal covering (6)
I believe the answer is:
patina
'covering' is the definition.
(patina is a kind of covering)
'old man securing a metal' is the wordplay.
'old man' becomes 'pa' (both can mean father).
'securing' means one lot of letters goes inside another.
'metal' becomes 'tin' (tin is a kind of metal**).
'a'+'tin'='atin'
'pa' placed around 'atin' is 'PATINA'.
(Other definitions for patina that I've seen before include "Developing surface film; sheen" , "Green gloss on old bronze" , "Film on copper or bronze" , "Surface film formed by age" , "Surface coating that comes with age" .)