Start eating most of finger, top of nail (3,2)
I believe the answer is:
dig in
'start eating' is the definition.
(I've seen this before)
'most of finger top of nail' is the wordplay.
'most of' means to remove the last letter.
'finger' becomes 'digit' ('digit' can be a synonym of 'finger').
'top of' says to take the initial letters.
The initial letter of 'nail' is 'n'.
'digit' with its final letter taken away is 'digi'.
'digi'+'n'='DIG IN'
(Other definitions for dig in that I've seen before include "Start to eat" , "Begin eating heartily" , "Enjoy the food" , "Start to scoff" , "Begin eating (informal)" .)