In a legal position inside the box, being attentive (11)
I believe the answer is:
considerate
'attentive' is the definition.
(thesaurus)
'in a legal position inside the box' is the wordplay.
'in a legal position' becomes 'onside' (I've seen this before).
'inside' indicates putting letters inside.
'the box' becomes 'crate' (crate is a kind of box).
'onside' put inside 'crate' is 'CONSIDERATE'.
'being' acts as a link.
(Other definitions for considerate that I've seen before include "Showing care" , "Showing concern for the feelings of others" , "Unselfish" , "Thoughtful for the feelings of others" , "Kind" .)