Druid oddly ineffective (3)
I believe the answer is:
dud
'ineffective' is the definition.
(similar in meaning)
'druid oddly' is the wordplay.
'oddly' means one should take alternating letters (letters in the odd-numbered positions are used).
The alternate letters of 'druid' are 'DUD'.
(Other definitions for dud that I've seen before include "Non-functioning thing" , "A thing failing to work" , "Palindrome for a flop" , "Bullet that fails to fire" , "A palindromic failure" .)