That's a joke about road 'improvement' - ultimately it's a safety issue (4,3)
I believe the answer is:
hard hat
'a safety issue' is the definition.
'that's a joke about road improvement ultimately' is the wordplay.
'that's a joke' becomes 'haha'.
'about' is an insertion indicator.
'road' becomes 'rd' (abbreviation).
'ultimately' says to take the final letters.
The last letter of 'improvement' is 't'.
'haha' going around 'rd' is 'hardha'.
'hardha'+'t'='HARD-HAT'
'it's' is the link.
(Other definitions for hard hat that I've seen before include "Protective helmet, like 25 ac" , "Protection for construction workers" , "Derby, perhaps" , "Headgear for building sites, etc" , "'nutcase'" .)