Legal volume needing a cover (5)
I believe the answer is:
valid
'legal' is the definition.
('valid' can be similar in meaning to 'legal')
'volume needing a cover' is the wordplay.
'volume' becomes 'v'.
'needing' means one lot of letters go next to another.
'cover' becomes 'lid' (lid is a kind of cover**).
'v'+'a'+'lid'='VALID'
(Other definitions for valid that I've seen before include "Sound, well-based" , "Officially acceptable" , "Effective" , "Legitimate" , "Binding" .)