Organised sales base to include seconds subject to tax (10)
I believe the answer is:
assessable
'subject to tax' is the definition.
(cable of being assessed for tax purposes)
'organised sales base to include seconds' is the wordplay.
'organised' is an anagram indicator.
'to include' means one lot of letters goes inside another.
'seconds' becomes 's'.
'sales'+'base'='salesbase'
'salesbase' anagrammed gives 'assesable'.
'assesable' going around 's' is 'ASSESSABLE'.
(Other definitions for assessable that I've seen before include "can be given a rating" , "Taxpayers will be" , "Can be judged" .)