Pack interior of streetcars (3)
I believe the answer is:
ram
'pack' is the definition.
(synonyms)
'interior of streetcars' is the wordplay.
'interior of' means to look at the middle letters.
'streetcars' becomes 'trams' ('tram' can be a synonym of 'streetcar').
The middle letters of 'trams' are 'RAM'.
(Other definitions for ram that I've seen before include "store for computers" , "Crash into, maybe for battering" , "man with many partners?" , "Butter" , "Aries is the Sign of the ..." .)