Start eating using a finger: shortly, any number (3,2)
I believe the answer is:
dig in
'start eating' is the definition.
(I've seen this before)
'a finger shortly any number' is the wordplay.
'a finger' becomes 'digit' (synonyms).
'shortly' means to remove the last letter.
'any number' becomes 'n' (mathematical symbol for an unspecified number).
'digit' with its last letter taken away is 'digi'.
'digi'+'n'='DIG IN'
'using' acts as a link.
(Other definitions for dig in that I've seen before include "Start eating (informal)" , "protect himself" , "Begin eating (informal)" , "Enjoy the food" , "Start to eat" .)