Romeo in Metro chaos is shaking (6)
I believe the answer is:
tremor
'shaking' is the definition.
('tremor' can be a synonym of 'shaking')
'romeo in metro chaos' is the wordplay.
'romeo' becomes 'r' (phonetic alphabet: alpha, bravo, charlie etc.).
'in' is an insertion indicator.
'chaos' indicates anagramming the letters.
'metro' with letters rearranged gives 'temor'.
'r' inserted inside 'temor' is 'TREMOR'.
'is' is the link.
(Other definitions for tremor that I've seen before include "Vibration from earthquake" , "Bit of excitement" , "A small shake, perhaps of earth ( 6)" , "Shiver in the earth" , "Involuntary shudder" .)