Nail excellent start (3,8)

I believe the answer is:

get cracking

'start' is the definition.
(to get cracking is to start or begin)

'nail excellent' is the wordplay.
'nail' becomes 'get' (both can mean to apprehend).
'excellent' becomes 'cracking'.
'get'+'cracking'='GET CRACKING'

(Other definitions for get cracking that I've seen before include "Prepare to break from shell" , "Act quickly and decisively" , "Start moving quickly" , "start flying" , "Bestir oneself" .)

I've seen this clue in The Times.

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