It's hard to provide safety on a building site (3)
I believe the answer is:
hat
'a building site' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'it's hard to provide safety on' is the wordplay.
I cannot quite see how this works, but
'hard' could be 'h' (abbreviation used in pencil classifications) and 'h' is located in the answer.
'on' could be 'at' (I've seen this in other clues) and 'at' is located in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for hat that I've seen before include "Head-covering" , "Cloche or fez?" , "Eg, bowler" , "Eg, derby" , "Lid" .)