Only packed as a precaution (4,2,4)
I believe the answer is:
just in case
'as a precaution' is the definition.
(I've seen this before)
'only packed' is the wordplay.
'only' becomes 'just' (synonyms).
'packed' becomes 'in case' (I've seen this before).
'just'+'incase'='JUST IN CASE'
(Other definitions for just in case that I've seen before include "Lest" , "as a precaution" , "the judge needs to be" , "In the unlikely event of problems" , "For insurance" .)