Shock as river fills crumbling Metro (6)
I believe the answer is:
tremor
'shock' is the definition.
(I have seen 'Small shock ' mean 'tremor' so perhaps 'shock' could also mean 'tremor')
'river fills crumbling metro' is the wordplay.
'river' becomes 'r' (abbreviation for river).
'fills' indicates putting letters inside.
'crumbling' indicates an anagram.
'metro' is an anagram of 'temor'.
'r' put inside 'temor' is 'TREMOR'.
'as' is the link.
(Other definitions for tremor that I've seen before include "make one shake" , "A slight shake, body or earth" , "Vibration - shaking" , "Small earthquake" , "Bit of excitement" .)