Man's hard hat outside (4)
I believe the answer is:
chap
'man's' is the definition.
(I've seen this in other clues)
'hard hat outside' is the wordplay.
'hard' becomes 'h'.
'hat' becomes 'cap' (I've seen this before).
'outside' is an insertion indicator.
'h' going within 'cap' is 'CHAP'.
(Other definitions for chap that I've seen before include "Cracked patch (of skin) - gentleman" , "Bloke, fellow" , "Split in skin" , "Cracked skin patch caused by cold" , "cove" .)