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Difficult nutcases? You see them on building sites! (4,4)

Ross

I believe the answer is:

hard hats

'you see them on building sites' is the definition.
(hard hats are worn on building sites)

'difficult nutcases?' is the wordplay.
'difficult' becomes 'hard' (synonyms).
'nutcases?' becomes 'hats' (nut can mean one's head).
'hard'+'hats'='HARD HATS'

I've seen this clue in The Times.
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