Barrage balloons finally seen on Oval ground (5)
I believe the answer is:
salvo
'barrage' is the definition.
(thesaurus)
'balloons finally seen on oval ground' is the wordplay.
'finally' indicates one should take the final letters.
'seen on' means one lot of letters go next to another.
'ground' indicates an anagram (grind the letters ).
The final letter of 'balloons' is 's'.
'oval' is an anagram of 'alvo'.
's'+'alvo'='SALVO'
(Other definitions for salvo that I've seen before include "A number of shots" , "Sudden round of applause" , "Rounds delivered" , "Barrage" , "Artillery salute" .)