Small earthquake damaged Metro railway primarily (6)
I believe the answer is:
tremor
'small earthquake' is the definition.
(I've seen this in another clue)
'damaged metro railway primarily' is the wordplay.
'damaged' is an anagram indicator.
'primarily' suggests taking the first letters.
The initial letter of 'railway' is 'r'.
'metro' is an anagram of 'tremo'.
'tremo'+'r'='TREMOR'
(Other definitions for tremor that I've seen before include "A slight shake, perhaps of earth" , "feeling of excitement" , "Quavering" , "A slight shake, body or earth" , "Vibration - shaking" .)