Hard to divide irrational number? Here's letter (3)
I believe the answer is:
phi
'letter' is the definition.
(phi is a kind of letter)
'hard to divide irrational number?' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'to divide' means one lot of letters goes inside another (some letters create a division in others).
'irrational number?' becomes 'pi' (pi is a kind of irrational number).
'h' going into 'pi' is 'PHI'.
'here's' is the link.
(Other definitions for phi that I've seen before include "Greek letter, spoken as f" , "classic character" , "Greek letter; sounds like expression of outrage" , "Greek letter, equivalent to F" , "21st Greek letter" .)