Dirty distressed pullet eats nothing (7)
I believe the answer is:
pollute
'dirty' is the definition.
(polluting is a kind of dirtying)
'distressed pullet eats nothing' is the wordplay.
'distressed' indicates an anagram.
'eats' is an insertion indicator.
'nothing' becomes 'O' (looks like zero - 0).
'pullet' with letters rearranged gives 'pllute'.
'pllute' going around 'o' is 'POLLUTE'.
(Other definitions for pollute that I've seen before include "Add impurities" , "Adulterate, contaminate" , "Make dirty or impure" , "Contaminate; defile" , "inhibit plant life?" .)