Old penny, one about to become invalid (6)
I believe the answer is:
expire
'become invalid' is the definition.
(I've seen this in another clue)
'old penny one about' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'penny' becomes 'p' (short for pence).
'one' becomes 'i' (Roman numeral).
'about' becomes 're' (regarding).
'ex'+'p'+'i'+'re'='EXPIRE'
'to' is the link.
(Other definitions for expire that I've seen before include "Come to an end; die" , "Die or go out of date" , "Die or lapse" , "Run out, exhale" , "Lose validity" .)