Poke it with a finger (5)
I believe the answer is:
digit
'a finger' is the definition.
(I know that finger can be written as digit)
'poke it' is the wordplay.
'poke' becomes 'dig' (synonyms).
'dig'+'it'='DIGIT'
'with' is the link.
(Other definitions for digit that I've seen before include "finger, perhaps" , "that's on hand" , "A small number" , "One's on hand" , "Single number" .)