Join in an emergency (5)
I believe the answer is:
merge
'join' is the definition.
(I've seen this before)
'in an emergency' is the wordplay.
'in' indicates a hidden word.
'MERGE' is hidden within 'emergency'.
(Other definitions for merge that I've seen before include "Fuse" , "Combine, blend" , "Make one" , "Join into one" , "Come together as one" .)