Not good to be cut by head of rusty nail (4)
I believe the answer is:
brad
'nail' is the definition.
(brad is a kind of nail)
'not good to be cut by head of rusty' is the wordplay.
'not good' becomes 'bad' ('bad' is the opposite to 'good').
'to be cut by' indicates putting letters inside.
'head of' indicates taking the first letters.
The first letter of 'rusty' is 'r'.
'bad' enclosing 'r' is 'BRAD'.
(Other definitions for brad that I've seen before include "Small-headed nail" , "Small tapered nail" , "Fixer" , "It has a point" , "One to get hammered" .)