Wearing box, as a precaution (2,4)
I believe the answer is:
in case
'as a precaution' is the definition.
(I've seen this before)
'wearing box' is the wordplay.
'wearing' becomes 'in' (wear in (and 'in' is fashionable)).
'box' becomes 'case' (I've seen this before).
'in'+'case'='IN CASE'
(Other definitions for in case that I've seen before include "As a precaution" , "for emergency" , "In order to allow for eventualities" , "Packed" , "Where traveller may keep clothes" .)