Small oval, disguised as a volley (5)
I believe the answer is:
salvo
'a volley' is the definition.
(I know that volley can be written as salvo)
'small oval disguised' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'disguised' is an anagram indicator.
'oval' is an anagram of 'alvo'.
's'+'alvo'='SALVO'
'as' is the link.
(Other definitions for salvo that I've seen before include "Simultaneous discharge of fire" , "Simultaneous discharge of guns" , "Ceremonial shots" , "Unusual excuse" , "A simultaneous salute of guns" .)