It's time unit of key personnel stood on two legs! (7)

'it's time unit' is the definition.
(the chronon is a hypothetical unit of time)

'key personnel stood on two legs' is the wordplay.
'two legs' means two replacements for 'leg'.
'key' becomes 'c' (musical key).
'personnel' becomes 'HR' (Human Resources).
'stood on' says to put letters next to each other (in a down clue, some letters go above others).
'leg' becomes 'on' (cricket term, fielding positions).
'leg' becomes 'on' (cricket term, fielding positions).
'c'+'hr'+'on'+'on'='CHRONON'

'of' is the link.