Positive shape in crate (7)
I believe the answer is:
certain
'positive' is the definition.
('certain' can be similar in meaning to 'positive')
'shape in crate' is the wordplay.
'shape' indicates anagramming the letters.
'in'+'crate'='incrate'
'incrate' with letters rearranged gives 'CERTAIN'.
(Other definitions for certain that I've seen before include "Quite sure, positive" , "Proved beyond doubt" , "Indisputable" , "Sure; specific" , "bound" .)