Hard to get inside pen: twist (6)
I believe the answer is:
writhe
'twist' is the definition.
(I know that twist can be written as writhe)
'hard to get inside pen' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'to get inside' indicates putting letters inside.
'pen' becomes 'write' (synonyms).
'h' going into 'write' is 'WRITHE'.
(Other definitions for writhe that I've seen before include "Painfully move" , "feel acute embarrassment" , "6, maybe in agony" , "worm" , "'Twist, squirm (6)'" .)