Nasty United Nations petitions want leader to be removed (10)
I believe the answer is:
unpleasant
'nasty' is the definition.
(similar in meaning)
'united nations petitions want leader to be removed' is the wordplay.
'united nations' becomes 'UN' (synonyms).
'petitions' becomes 'pleas'.
'leader to be removed' means to remove the first letter.
'want' with its first letter taken off is 'ant'.
'un'+'pleas'+'ant'='UNPLEASANT'
(Other definitions for unpleasant that I've seen before include "like Mrs Warren's Profession?" , "Not nice" , "Obnoxious" , "Troublesome" , "Nasty" .)