There's nothing in wild deer to have a bad effect (5)
I believe the answer is:
erode
'a bad effect' is the definition.
Although both the answer and definition are verbs in their base form, I can't see how they can define each other.
'there's nothing in wild deer' is the wordplay.
'there's nothing' becomes 'o' (I've seen this in other clues).
'in' means one lot of letters goes inside another.
'wild' is an anagram indicator.
'deer' is an anagram of 'erde'.
'o' going inside 'erde' is 'ERODE'.
'to have' is the link.
Can you help me to learn more?
(Other definitions for erode that I've seen before include "Wear away with weather" , "Gradually wear away" , "Slowly remove" , "fragment" , "Become ground down or deteriorate" .)