A storehouse lacking nothing is proficient (5)
I believe the answer is:
adept
'proficient' is the definition.
('adept' can be a synonym of 'proficient')
'a storehouse lacking nothing' is the wordplay.
'storehouse' becomes 'depot' ('depot' can be a synonym of 'storehouse').
'lacking' indicates named letters should be taken away.
'nothing' becomes 'o' (looks like zero - 0).
'depot' with 'o' taken out is 'dept'.
'a'+'dept'='ADEPT'
'is' acts as a link.
(Other definitions for adept that I've seen before include "Very skilled/proficient" , "clever" , "Skilful and adroit" , "One highly skilled could be taped" , "Proficient, skilled" .)