Rotten - dad twisted and came first! (6)
I believe the answer is:
addled
'rotten' is the definition.
(I've seen this before)
'dad twisted and came first' is the wordplay.
'twisted' indicates anagramming the letters.
'and' means one lot of letters go next to another.
'came first' becomes 'led' (to lead is to go first).
'dad' anagrammed gives 'add'.
'add'+'led'='ADDLED'
(Other definitions for addled that I've seen before include "Foolish or confused like bad egg" , "turned bad" , "Gone bad (of eggs)" , "Dad led (anag.)" , "Confused (wits); made rotten" .)