In case of fire, move fast! (5)
I believe the answer is:
shoot
'fire move fast' is the definition.
'shoot' can be an answer for 'fire' (shooting is a kind of firing). I am unsure of the 'move fast' bit.
'in case of' is the wordplay.
'in' is an insertion indicator.
'case' becomes 'shot' (I can't justify this - if you can you should believe this answer much more).
'of' becomes 'o''.
'shot' going around 'o' is 'SHOOT'.
Can you help me to learn more?
(Other definitions for shoot that I've seen before include "New growth; last word of Hamlet" , "Use a gun; film" , "Fire at the new branch?" , "Say what you have to say" , "modelling session" .)