First part of rust in rotten nail (4)
I believe the answer is:
brad
'nail' is the definition.
(brad is a kind of nail)
'first part of rust in rotten' is the wordplay.
'first part of' says to take the initial letters.
'in' indicates putting letters inside.
'rotten' becomes 'bad' ('bad' can be similar in meaning to 'rotten'**).
The initial letter of 'rust' is 'r'.
'r' placed within 'bad' is 'BRAD'.
(Other definitions for brad that I've seen before include "-- Pitt, US actor" , "Fixer" , "Type of nail" , "One may secure" , "Small tapering nail" .)