In case of earthquake cross river (3)
I believe the answer is:
exe
'river' is the definition.
(I've seen this before)
'in case of earthquake cross' is the wordplay.
'in' means one lot of letters goes inside another.
'case of' says to hollow out the word (remove centre letters) (outsides of).
'cross' becomes 'x'.
'earthquake' with its middle removed is 'ee'.
'ee' enclosing 'x' is 'EXE'.
(Other definitions for exe that I've seen before include "source of water" , "River flowing from Exmoor to Exmouth via Exeter" , "Somerset/Devon river" , "Banker" , "River of southwest England" .)