Old man, head on verge of retirement (5)
I believe the answer is:
pater
'old man' is the definition.
('pater' can be a synonym of 'old man')
'head on verge of retirement' is the wordplay.
'head' becomes 'pate' (I've seen this before**).
'on' means one lot of letters go next to another.
'verge of' suggests taking the first letters.
The first letter of 'retirement' is 'r'.
'pate'+'r'='PATER'
(Other definitions for pater that I've seen before include "A posh father, from Latin" , "Our Father - . . . . . Noster" , "Dad (Latin)" , "Walter --, English essayist; father" , "Taper away like father" .)