Completely impractical to get herringbone trousers (10)
I believe the answer is:
altogether
'completely' is the definition.
(I know that completely can be written as altogether)
'impractical to get herringbone trousers' is the wordplay.
'trousers' indicates a hidden word (trouser can mean to steal or take).
'ALTOGETHER' is hidden within 'impractical to get herringbone'.
(Other definitions for altogether that I've seen before include "special to get Hermes trousers" , "Wholly" , "Completely, on the whole" , "Entirely" , "nude in this" .)