Lose job when you grab the trunk (3,3,4)
I believe the answer is:
get the boot
'lose job' is the definition.
I know nothing about this answer so I can't tell whether this works.
'you grab the trunk' is the wordplay.
'you grab' becomes 'get' (I am not sure about this - if you are sure you should believe this answer much more).
'trunk' becomes 'boot' (synonyms).
'get'+'the'+'boot'='GET THE BOOT'
'when' acts as a link.
Can you help me to learn more?
(Other definitions for get the boot that I've seen before include "To be fired" , "Be dismissed from job (colloq.)" , "be dropped" .)