Digit hard to get into apple, say (5)
I believe the answer is:
three
'digit' is the definition.
(I know that three is a type of digit)
'hard to get into apple say' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'to get into' means one lot of letters goes inside another.
'apple say' becomes 'tree' (apple is a kind of tree).
'h' going into 'tree' is 'THREE'.
(Other definitions for three that I've seen before include "Subject of 1 9" , "Cardinal number" , "Number of Dumas's musketeers" , "Number of blind mice" , "Number of Furies or Norns" .)