Difficult to laugh on the way (4)
I believe the answer is:
hard
'difficult' is the definition.
(I know that difficult can be written as hard)
'laugh on the way' is the wordplay.
'laugh' becomes 'ha'.
'on' means one lot of letters go next to another.
'the way' becomes 'rd' (short for road).
'ha'+'rd'='HARD'
'to' acts as a link.
(Other definitions for hard that I've seen before include "Solid; tough" , "Difficult; severe" , "Novel featuring Mr Gradgrind" , "badly" , "Tough; rigid" .)