Five get a cover that's sound (5)
I believe the answer is:
valid
'that's sound' is the definition.
(similar in meaning)
'five get a cover' is the wordplay.
'five' becomes 'V' (Roman numeral).
'get' is a charade indicator (letters next to each other).
'cover' becomes 'lid' (lid is a kind of cover).
'v'+'a'+'lid'='VALID'
(Other definitions for valid that I've seen before include "Based on reason or law" , "Legitimate" , "Legally good" , "Legal, authentic" , "Officially acceptable" .)