Nothing to do with millions in retirement for old King's Counsel (6)
I believe the answer is:
merlin
'old king's counsel' is the definition.
(advisor to King Arthur)
'nothing to do with millions in retirement' is the wordplay.
'nothing' becomes 'nil' (eg nil is zero in football).
'to do with' becomes 're' (regarding).
'millions' becomes 'm' (abbreviation).
'in retirement' is a reversal indicator (retire can mean to go back or backwards).
'nil'+'re'+'m'='nilrem'
'nilrem' backwards is 'MERLIN'.
'for' is the link.
(Other definitions for merlin that I've seen before include "Arthurian wizard" , "Flighty type" , "King Arthur's magician" , "Arthur's sorcerer" , "Kind of hawk or Camelot's magician" .)