Secret compartment (odd italics) (10)
I believe the answer is:
cabalistic
'secret' is the definition.
('cabalistic' can be a synonym of 'secret')
'compartment odd italics' is the wordplay.
'compartment' becomes 'cab' (cab is a kind of compartment).
'odd' indicates anagramming the letters.
'italics' is an anagram of 'alistic'.
'cab'+'alistic'='CABALISTIC'
(Other definitions for cabalistic that I've seen before include "Having a hidden meaning" , "Secret" .)