However difficult, hard going in (6)
I believe the answer is:
though
'however' is the definition.
(synonyms)
'difficult hard going in' is the wordplay.
'difficult' becomes 'tough' (similar in meaning).
'hard' becomes 'h' (abbreviation used in pencil classifications).
'going in' means one lot of letters goes inside another.
'tough' enclosing 'h' is 'THOUGH'.
(Other definitions for though that I've seen before include "However" , "Even if" , "Despite the fact that" , "Concessive conjuction" , "Nevertheless" .)