Join in emergency (5)
I believe the answer is:
merge
'join' is the definition.
(I've seen this before)
'in emergency' is the wordplay.
'in' indicates the answer is hidden within the clue.
'MERGE' can be found hidden inside 'emergency'.
(Other definitions for merge that I've seen before include "Combine, coalesce" , "Come together as one" , "Become combined" , "Amalgamate" , "Mix" .)