Like a shot that’s one foot off (5,4)
I believe the answer is:
right away
'like a shot' is the definition.
(both can mean immediately)
'one foot off' is the wordplay.
'one foot' becomes 'right' (right foot).
'off' becomes 'away' (synonyms).
'right'+'away'='RIGHT AWAY'
'that's' is the link.
(Other definitions for right away that I've seen before include "without hesitation" , "Immediately" , "This instant, without delay" , "in no time" , "This very minute, without delay" .)