Cheers military vehicles encircling Hungary (6)
I believe the answer is:
thanks
'cheers' is the definition.
('cheers' can informally mean 'thank you')
'military vehicles encircling hungary' is the wordplay.
'military vehicles' becomes 'tanks' (tank is a kind of military vehicle).
'encircling' indicates putting letters inside.
'hungary' becomes 'H' (abbreviation used in vehicle registration codes).
'tanks' enclosing 'h' is 'THANKS'.
(Other definitions for thanks that I've seen before include "recognition?" , "Word of appreciation" , "I appreciate that" , "Much obliged!" , "Cheers" .)