Little support over location of UN headquarters (5)
I believe the answer is:
teeny
'little' is the definition.
(similar in meaning)
'support over location of un headquarters' is the wordplay.
'support' becomes 'tee' (tee is a kind of support).
'over' says to put letters next to each other.
'location of un headquarters' becomes 'ny' (the UN's headquarters is in New York).
'tee'+'ny'='TEENY'
(Other definitions for teeny that I've seen before include "Minuscule" , "Very small (informal)" , "Extremely small" , "Very little relating" , "Microscopic" .)