Firstly tap on elfin digit (3)
I believe the answer is:
toe
'digit' is the definition.
(digit on a foot)
'firstly tap on elfin' is the wordplay.
'firstly' suggests taking the first letters.
The initial letters of 'tap on elfin' is 'TOE'.
(Other definitions for toe that I've seen before include "Extremity of the body" , "One's nailed" , "Conform-...the line" , "Small member" , "Lower tip of something" .)